PA Infotypes contain many fields that are not "used", at least not by the users, and there could be some variations according to the Country Code. Furthermore, some Infotypes are Country dependant.
As for OM Infotypes, their list will vary according to the Object being maintained, and there could be Country specific variations.
If they are an SAP vendor, I would expect them to already have a very good knowledge of standard SAP Infotypes and their fields, and the Country specific Infotypes or variations. Most training manuals already detail the important and standard fields.
If you query your system (through se16n or ad-hoc query), you should be able to tell them what Infotypes and Fields you currently use. For example, you could tell them that you use IT0008 but only the first 2 possible WT lines (you should not need to tell them that there are 18 unused lines).