
Hello marclabru,
This is the excerpt from the documentation ("Design tools | View Custom Organ Design Module ... format documentation" on the main menu) for that attribute:
"The version of Hauptwerk for which the file was created. Hauptwerk uses this value to ensure that a custom organ definition file is compatible with the version of Hauptwerk installed by a user, and whether the user needs to upgrade Hauptwerk or to migrate the custom organ definition file to a later format."
If you use Hauptwerk's built-in CODM SQLite database import function to save the CODM ODF, Hauptwerk will automatically set that attribute to the current version of Hauptwerk that you're using, since the file will be saved in that format.
If you manually set it to a lower version (e.g. 5) than the version you're using (e.g. 7) then you need to make certain that the file is truly in the format required by the lower version, otherwise it may not load or work properly in the earlier version. If you search the above document for each major version number above the version you want to target (e.g. 5) up to the version you're using (e.g. 7) by prefixing each version with 'HWv', you can check whether there were any format changes in that major version.
Hence if you're using Hauptwerk v7 but want to target v5 then search the document for "HWv6" and "HWv7". Trying those searches myself, I see that only one attribute was changed between v5 and v7: "_General.UniqueOrganID", and the only change to that was that it it now allows the full range of values so that commercial producers can use values outside of the 800000-899999 'personal/testing' range.
Having checked that the file's format is indeed valid for the specified version, do make sure that someone actually tries it on the target version (e.g. v5), for good measure.