Beginning with 4D V6 Sets can be stored on the server. Sets have the potential of becoming quite large and using considerable memory. If you are going to be using several large sets for INTERSECTION, UNION, and DIFFERENCE for the purpose of printing long reports, and these sets are going to exist throughout the report to be used over and over in different combinations, this could tie up quite a bit of the server's resources. It may be preferable to make these sets local sets, moving the set to the client where they only take up client memory.
A local Set is a set created and maintained in memory locally on the client, not the server. A Local/Client set is identified by a dollar sign "$" preceding its name.