SourceAnyWhere is licensed on a per user basis (not by concurrent users). For example, if you have three people who use SourceAnywhere for VSS, you need to purchase three licenses: one license for each of those three people. Each user may install the SourceAnywhere for VSS Client on multiple machines as needed.

For information on the differences between the Standard and Professional version please see the "SourceAnywhere for VSS Standard and Professional Feature Matrix" in the "Evals & Downloads" section.