Database Version
BigBusiness updates the built-in database it uses for security and fixes when available.
Within a major version like Big Business 14 this might happen a few times in Updates. Usually these do not require changing Clients, just Server, unless the "fix" is in Clients.
On the other hand, a new major version like BigBusiness 15 is a new database entirely. An Upgrade entails replacing all Clients, which may only work with newest Systems.
Related: