The XIMSS API was introduced by CommuniGate Systems in their CommuniGate Pro (often referred to as "CGP") server starting with version 5.1 in Summer 2006. Their announcement described XIMSS as being based on the "open, proposed standard XIMSS, based on previous XML RFCs" but it is not clear whether there is any formal proposal defining XIMSS before any standards body, and CommuniGate does not explicitly specify which RFC's they were referring to as a basis for XIMSS. The only server implementation of XIMSS is CommuniGate Pro, and there are few applications publicly available using XIMSS other than CommuniGate's Flash-based "Pronto" integrated messaging client. It is not explicitly clear in any documentation  whether XIMSS can be freely implemented in other servers or clients without permission from CommuniGate, but their description of it as "open" implies some level of freedom.
The expressed purpose of XIMSS is to offer a single client interface based on widely-familiar open technologies (i.e. HTTP and XML) to a complex of communications services (email, calendaring, IM, VoIP, etc.) provided by one server. As specified, XIMSS is not intrinsically bound to CGP as a server, however its specified functional scope is clearly inspired by the CGP feature set.