Is there a spec document of the Smart Zoom Directory file (SZD) format?

[PolCPP]

Hi, the spec document mentions this

For serving the tiles, the server program should not – for each tile request – read the ZIP with a
regular ZIP library, but write a map file once before the first served tile. Reading an entire ZIP
directory for each tile, which has to be served (of a ZIP file with possibly millions of contained tiles) is too inefficient. Creating a map file, considerably speeds up the process. The design of such a map, the Smart Zoom Directory file (SZD) will be discussed later and is not important for just outputting SZI files, e. g. from a scanner

Is that SZD format described anywhere? Can't seem to find any info about it.