Mapsets represent a set of reusable mappings. If you have a table that appears in multiple message table grammars, you might want to use the same mappings since this will minimize the amount of maintenance you need to perform to keep the VMD file current. For instance, a mapset that is shared by several table grammars will only have to be changed once for every alteration.

Conversely, if you want to have different mappings for each table, you can create a different mapset for each use of a particular table. This still minimizes the amount of maintenance for you because having different mapsets for a table means you still only have to maintain one VMD file, instead of two.