As I understood it long ago and far away, EACH drawing sheet has its own revision letter... but Sheet 1 was always the lead-sheet for revisions... and has the highest Rev letter.
The first sheet could be Rev X but that Rev incremented when any of the subsequent sheets were changed or added. This was because changes regardless of whether they were on Sheet 1... were briefly recorded on sheet 1... and usually initialed-off. This was true for very old hand drawings... they could have many things cluttered about. So, drawing sheet 1 would have the latest REV-letter spanning all sheets of the drawing for procurement. A bit confusing... and I could be wrong.
Simply refer to drawing sheet 1 for the entire drawing package rev letter [even though other sheets usually have their own rev]. However since each sheet could have a unique Rev letter... that is the way to ensure You have the latest version of all drawing sheets... or not.
Of course... I've seen lots of crazy stuff in drawing packages submitted to vendors on contracts [all companies]. Especially... when there could be military and civilian assemblies, virtually identical, but incorporated under one drawing number with unique identifiers that prohibit their use on the 'other' type Acft, such as identical Assys... but different part numbers... simply because of different finishes on the parts [hydrocarbon resistant hydraulic oil... or Skydrol resistant.
Weirdly I have see segregated Boeing drawings under one drawing number affecting early 707s, KC-135s and 720s.
My head always hurt.
CATIA makes it a lot simpler to view parts and assemblies in what ever specific configuration selected.