Looking at the doc my understanding is these codes are used if you have several different AFCADs for an airport but I'm still not 100% sure what this means when downloading a file from here.
Yes, the random characters are added automatically by the editor, so if you have different AFCADs from the same airport, all your customizations can coexists so, whenever you switch AFCAD, its own customization file will be loaded.
Shared files might have been created with an earlier version of the editor (that didn't had this feature) or their original author might have simply removed the random characters before the upload.
What the characters are doesn't matter, as long as you have ICAO-xxxxxxxx, it will work so, if you want to rename a file such as EDDM-Aerosoft_with_static_airplanes.INI, because it would be clearer for you, that's fine, the only mandatory part are the first 4 characters of the ICAO and the dash.
And, the .INI name doesn't have any relationship with the name of the AFCAD that customization was based on, because the AFCAD name is stored inside the .INI file itself.