diff --git a/docs/index.md b/docs/index.md
index a9b20c652..cdd865469 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -13,6 +13,29 @@ validating clinical trial data against CDISC data standards.
 
 ---
 
+## Which Engine Should I Use?
+
+> **At present**, rule validation is split across two engines while work is underway to merge
+> the Verisian fork back into this repository. Until that merge is complete, please select the
+> engine based on the rules you're running:
+
+| If you're running…                          | Use…                                                                                                        |
+|---------------------------------------------|-------------------------------------------------------------------------------------------------------------|
+| **SDTM** or **FDA Business Rules**          | The **Verisian Engine** — [verisianHQ/cdisc-rules-engine](https://github.com/verisianHQ/cdisc-rules-engine) |
+| **TIG**, **USDM**, **ADaM**, or **SEND**    | The **CDISC Engine** (this repository), as described below                                                  |
+
+This split is temporary and reflects differing underlying architectures — the Verisian Engine is
+built on SQL, while the CDISC Engine (this repository) is built on pandas/dask/jsonata. These
+implementations will be harmonized over time, at which point this section will be
+updated to reflect the change.
+
+For setup and usage of the **Verisian Engine** (installation, environment configuration,
+CLI options, etc.), please follow the instructions in their
+[README](https://github.com/verisianHQ/cdisc-rules-engine/blob/main/README.md) rather than
+this one, since its setup steps (e.g. SQL/Postgres configuration) differ from those below.
+
+---
+
 ## Scope
 
 CORE validates study datasets against published CDISC conformance rules for the following
@@ -27,6 +50,11 @@ standards:
 | **FDA Business Rules** | FDA submission conformance rules                 |
 | **USDM**               | Unified Study Definitions Model                  |
 
+> **Note:** At present, SDTMIG and FDA Business Rules validation should be performed via the
+> **Verisian Engine** (see [Which Engine Should I Use?](#which-engine-should-i-use) above).
+> This repository currently handles **TIG**, **USDM**, and **SEND** validations, plus **ADaM**
+> unless/until noted otherwise.
+
 CORE validates data _structure and conformance_ against published rules. It is not a
 replacement for clinical review, statistical analysis, or submission readiness assessment.
 Rule logic is defined in [`cdisc-open-rules`](https://github.com/cdisc-org/cdisc-open-rules).
@@ -59,4 +87,3 @@ Rule logic is defined in [`cdisc-open-rules`](https://github.com/cdisc-org/cdisc
   [wiki.cdisc.org — Getting Started](https://wiki.cdisc.org/display/LIBSUPRT/Getting+Started%3A+Access+to+CDISC+Library+API+using+API+Key+Authentication)
 - **Published CDISC Conformance Rules GitHub:**
   [cdisc-open-rules](https://github.com/cdisc-org/cdisc-open-rules)
-  
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