The common usage selection displays every routine in the application where the common is declared. The report provides the common size (in bytes), a 'A-Z' key reflecting common composition, and whether any member in the common was set or referenced in the routine. A different key or size obviously denotes a change in the common between routines - and may indicate an error in your source. If the common was neither set nor referenced in a given routine, then the common need not be declared in that routine (particularly when it is not declared in an include file).
Clicking on the common label takes you to the (initial) common definition for that common. Clicking on Details takes you to the detailed common information for that common - see below.
The common details report lists the routines where every common member (more precisely, the common offset) is either set or referenced (either directly or via equivalence) throughout the entire application. The report is presented by offset in the common, in case common composition differs between functions. Obviously, if you are trying to trace the usage of a given common member, this is the report to use. The member name in each routine and the type are also displayed.
Clicking on the common label takes you to the (initial) common definition for that common.
The locations reported in the common detail report normally follow the same format as the Local report cross reference locations, unless the brief common detail report, (GREPT=D) is used, in which case only the notation 'set' or 'refd' is reported for each function.