Visual Source Safe API available with RTC software
One answer
As far as I know it is not an API but an interface that is provided. See https://jazz.net/downloads/rational-team-concert/releases/4.0.0.1/RTC-MSSCI-Client-repo-4.0.0.1.zip . MSSCC is defined by Microsoft. Any consumers of MSCC have to conform to that interface.
Comments
I should have added that the Jazz MSCCI implements the server side of the interface. The idea is that tools that support this interface, can use it and work against Jazz SCM. Since the interface is very ancient (I remember it being around 1995) it is widely supported by tools. Here is the help: http://pic.dhe.ibm.com/infocenter/clmhelp/v4r0/topic/com.ibm.team.scm.doc/topics/t_scm_msscci_config.html . Here some more information
https://jazz.net/library/article/1107
https://jazz.net/search_results.jsp?q=MSScci#page=0&type=type%3DDocument-WikiPage&q=MSScci
https://jazz.net/search_results.jsp?q=MSScci#page=0&type=type%3DDocument-ForumThread&q=MSScci