diff --git a/ReqIFSharp.Extensions.Tests/ReqIFExtensions/ExternalObjectExtensionsTestFixture.cs b/ReqIFSharp.Extensions.Tests/ReqIFExtensions/ExternalObjectExtensionsTestFixture.cs
new file mode 100644
index 0000000..99fc149
--- /dev/null
+++ b/ReqIFSharp.Extensions.Tests/ReqIFExtensions/ExternalObjectExtensionsTestFixture.cs
@@ -0,0 +1,87 @@
+//  -------------------------------------------------------------------------------------------------
+//  <copyright file="ExternalObjectExtensionsTestFixture.cs" company="Starion Group S.A.">
+//
+//    Copyright 2017-2025 Starion Group S.A.
+//
+//    Licensed under the Apache License, Version 2.0 (the "License");
+//    you may not use this file except in compliance with the License.
+//    You may obtain a copy of the License at
+//
+//        http://www.apache.org/licenses/LICENSE-2.0
+//
+//    Unless required by applicable law or agreed to in writing, software
+//    distributed under the License is distributed on an "AS IS" BASIS,
+//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//    See the License for the specific language governing permissions and
+//    limitations under the License.
+//
+//  </copyright>
+//  -------------------------------------------------------------------------------------------------
+
+namespace ReqIFSharp.Extensions.Tests.ReqIFExtensions
+{
+    using System;
+    using System.Text;
+
+    using NUnit.Framework;
+
+    using ReqIFSharp;
+    using ReqIFSharp.Extensions.ReqIFExtensions;
+
+    /// <summary>
+    /// Suite of tests for the <see cref="ExternalObjectExtensions"/> class
+    /// </summary>
+    [TestFixture]
+    public class ExternalObjectExtensionsTestFixture
+    {
+        [Test]
+        public void Verify_that_CreateUrl_throws_when_externalObject_is_null()
+        {
+            Assert.That(() => ExternalObjectExtensions.CreateUrl(null),
+                Throws.TypeOf<ArgumentNullException>().With.Property("ParamName").EqualTo("externalObject"));
+        }
+
+        [Test]
+        public void Verify_that_CreateUrl_returns_expected_url()
+        {
+            var reqif = new ReqIF();
+
+            var header = new ReqIFHeader
+            {
+                Identifier = "reqif-identifier",
+                DocumentRoot = reqif
+            };
+
+            reqif.TheHeader = header;
+
+            var coreContent = new ReqIFContent
+            {
+                DocumentRoot = reqif
+            };
+
+            reqif.CoreContent = coreContent;
+
+            var specObject = new SpecObject
+            {
+                ReqIFContent = coreContent
+            };
+
+            coreContent.SpecObjects.Add(specObject);
+
+            var attributeValueXhtml = new AttributeValueXHTML
+            {
+                SpecElAt = specObject
+            };
+
+            var externalObject = new ExternalObject(attributeValueXhtml)
+            {
+                Uri = "graphics/diagram.png"
+            };
+
+            var expectedBase64Uri = Convert.ToBase64String(Encoding.UTF8.GetBytes(externalObject.Uri));
+            var expectedUrl = $"/reqif/{header.Identifier}/externalobject/{expectedBase64Uri} ";
+
+            Assert.That(externalObject.CreateUrl(), Is.EqualTo(expectedUrl));
+        }
+    }
+}
diff --git a/ReqIFSharp.Extensions.Tests/ReqIFExtensions/IdentifiableExtensionsTestFixture.cs b/ReqIFSharp.Extensions.Tests/ReqIFExtensions/IdentifiableExtensionsTestFixture.cs
new file mode 100644
index 0000000..f9ae034
--- /dev/null
+++ b/ReqIFSharp.Extensions.Tests/ReqIFExtensions/IdentifiableExtensionsTestFixture.cs
@@ -0,0 +1,92 @@
+//  -------------------------------------------------------------------------------------------------
+//  <copyright file="IdentifiableExtensionsTestFixture.cs" company="Starion Group S.A.">
+//
+//    Copyright 2017-2025 Starion Group S.A.
+//
+//    Licensed under the Apache License, Version 2.0 (the "License");
+//    you may not use this file except in compliance with the License.
+//    You may obtain a copy of the License at
+//
+//        http://www.apache.org/licenses/LICENSE-2.0
+//
+//    Unless required by applicable law or agreed to in writing, software
+//    distributed under the License is distributed on an "AS IS" BASIS,
+//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//    See the License for the specific language governing permissions and
+//    limitations under the License.
+//
+//  </copyright>
+//  -------------------------------------------------------------------------------------------------
+
+namespace ReqIFSharp.Extensions.Tests.ReqIFExtensions
+{
+    using System;
+    using System.Collections.Generic;
+
+    using NUnit.Framework;
+
+    using ReqIFSharp;
+    using ReqIFSharp.Extensions.ReqIFExtensions;
+
+    /// <summary>
+    /// Suite of tests for the <see cref="IdentifiableExtensions"/> class
+    /// </summary>
+    [TestFixture]
+    public class IdentifiableExtensionsTestFixture
+    {
+        private static IEnumerable<TestCaseData> QueryTypeDisplayNameTestCases()
+        {
+            yield return new TestCaseData(new AttributeDefinitionBoolean(), "Boolean Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionDate(), "Date Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionEnumeration(), "Enumeration Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionInteger(), "Integer Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionReal(), "Real Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionString(), "String Attribute Definition");
+            yield return new TestCaseData(new AttributeDefinitionXHTML(), "XHTML Attribute Definition");
+            yield return new TestCaseData(new DatatypeDefinitionBoolean(), "Boolean Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionDate(), "Date Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionEnumeration(), "Enumeration Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionInteger(), "Integer Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionReal(), "Real Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionString(), "String Datatype Definition");
+            yield return new TestCaseData(new DatatypeDefinitionXHTML(), "XHTML Datatype Definition");
+            yield return new TestCaseData(new EnumValue(), "Enum Value");
+            yield return new TestCaseData(new RelationGroup(), "Relation Group");
+            yield return new TestCaseData(new SpecHierarchy(), "Spec Hierarchy");
+            yield return new TestCaseData(new Specification(), "Specification");
+            yield return new TestCaseData(new SpecObject(), "Spec Object");
+            yield return new TestCaseData(new SpecRelation(), "Spec Relation");
+            yield return new TestCaseData(new RelationGroupType(), "Relation Group Type");
+            yield return new TestCaseData(new SpecificationType(), "Specification Type");
+            yield return new TestCaseData(new SpecObjectType(), "Spec Object Type");
+            yield return new TestCaseData(new SpecRelationType(), "Spec Relation Type");
+        }
+
+        [Test]
+        [TestCaseSource(nameof(QueryTypeDisplayNameTestCases))]
+        public void Verify_that_QueryTypeDisplayName_returns_expected_display_name(Identifiable identifiable, string expected)
+        {
+            Assert.That(identifiable.QueryTypeDisplayName(), Is.EqualTo(expected));
+        }
+
+        [Test]
+        public void Verify_that_QueryTypeDisplayName_throws_when_identifiable_is_null()
+        {
+            Assert.That(() => IdentifiableExtensions.QueryTypeDisplayName(null),
+                Throws.TypeOf<ArgumentNullException>().With.Property("ParamName").EqualTo("identifiable"));
+        }
+
+        [Test]
+        public void Verify_that_QueryTypeDisplayName_throws_for_unknown_identifiable_type()
+        {
+            var identifiable = new UnknownIdentifiable();
+
+            Assert.That(() => identifiable.QueryTypeDisplayName(),
+                Throws.TypeOf<InvalidOperationException>().With.Message.Contains(nameof(UnknownIdentifiable)));
+        }
+
+        private class UnknownIdentifiable : Identifiable
+        {
+        }
+    }
+}