A word on current, forward & backward compatibility –
Traditionally StarTeam clients are forward compatible with later versions of the server & backward compatible with earlier versions of the server. Similarly, StarTeam servers are forward compatible with later versions of the client & backward compatible with earlier versions of the client.
There is a cost to compatibility. An earlier client is unaware of features offered by a later server, and a later client is capable of using &/or surfacing only those features offered by an earlier server.
For example, a 2009 client has to check content into a 2005R2 server every time, whether or not the server already has the content. Similarly a 2005R2 client has to check content into a 2009R2 server every time, even though the server already has the content. This is because the newer server has a feature that the older client is unaware of, (in this case, the ability to check for content by MD5 before streaming it over) and the newer client cannot use a feature that the older server does not offer.
Ideal compatibility, i.e. the perfect match is achieved between a client & server paired to the same release. e.g. 2005R2 client to 2005R2 server, 2009 client to 2009R2 server; 12.5 client to 12.5 server, etc. both client & server are tuned to the same feature set, and optimally positioned to take advantage of that set.
Forward & backward compatibility is typically retained for one or two client/server versions in either direction. StarTeam provides this compatibility to facilitate upgrades, with minimal downtime; for instance, first upgrade the server, then upgrade clients. Since there are typically thousands of client seats to be upgraded, this can take time.
While it is possible to indefinitely run earlier clients against latter servers or vice versa, StarTeam engineering does not recommend this practice.