I have noticed most instrument departures have a number after them. TRUKN2 GAP7 CITY3 for example at KSFO. Do the numbers represent a revision of say TRUNK1 GAP6 CITY3 for maybe a change in and altitude due to construction of a building, or FAA SID Departure criteria? Thanks in advance!
Yes, that is correct. The number indicates a version of the procedure, and runs from
9 before cycling back to
1. The changes may be due to newly-surveyed obstructions, or changes in ATC procedures to allow for more efficient traffic flow, or the decommissioning of conventional NAVAIDs (VORs) which used to be part of a procedure.
The FAA publishes documents highlighting the changes between successive versions of procedures, for example this one shows the changes between the FROGZ TWO and FROGZ THREE arrivals. These and other terminal procedures can be accessed at the Terminal Procedures site. Third parties like SkyVector also show these documents.