What I found out is that the "Custom" privilege means that permissions have been configured in a way that are more complex than is provided and can be shown in the basic user settings of the Get Info window (namely: Read, Write, No Access).
If I query the volume in Terminal with
ls -leda /Volumes/Some\ HD/ I get something like this. Option
e is to print the ACL associated with the file (or directory), if present, in long (
drwx------+ 18 Alex staff 680 1 Oct 18:19 /Volumes/Some\ HD/
0: group:com.apple.sharepoint.group.6 allow search
1: group:com.apple.sharepoint.group.5 allow search
2: D93ABBA3-3566-4A08-878E-B2E7C575B4A6 allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity
I just still don't know where the com.apple.sharepoint.group users are coming from and
D93ABBA3-3566-4A08-878E-B2E7C575B4A6 looks a bit cryptic if that's me.