A "data subset" is defined as the subset of data described by one single application of the collection of descriptors. In this context, the "collection of descriptors" means ALL the descriptors included in section 3 of the BUFR message

A "data subset" is defined as the subset of data described by one single application of the collection of descriptors. In this context, the "collection of descriptors" means ALL the descriptors included in section 3 of the BUFR message.
In other words, one pass through the complete collection of descriptors will allow one to decode one data subset from section 4. One then loops back in the descriptor list for as many times as indicated by the number of data subsets (key "numberOfSubsets"). All the data in section 4 are properly described by repeated use of the same set of descriptors from section 3.