How does the "Belgian" NeTEx standard differ from the "normal" NeTEx?
Belgian NeTEx, as developed by OSLO, is a semantic (URI-based) version of the generic European NeTEx specification. It includes a common vocabulary, as well as three application profiles (APs): 'Time table', 'vehicle scheduling' and 'stop place'.
The Belgian NeTEx was specifically developed to meet the different needs of Belgian rail, bus, tram and metro networks. The Belgian NeTEx profile covers parts 1, 2 and 4 of the European NeTEx specification (but does not include part 3 (fares) nor parts 5 and 6).
On Github you can find more detailed information the Belgian NeTEx:
https://github.com/belgif/thematic/blob/master/models/public%20transport/README.md