the MSSCCI package.
One answer
I'm not sure I'm following the question exactly, but since RTC 1.0, there has been both Eclipse and Visual Studio plugins to work as RTC clients (Note: These capabilities go beyond SCM (which is what MSSCCI focuses on), such as Work Item, Build, Process management, etc.). Neither of these were designed around the MS-SCCI APIs.
Help doc for visual studio: https://www.ibm.com/docs/en/elm/7.0.2?topic=mcr-getting-started-engineering-workflow-management-client-microsoft-visual-studio-ide
That being said, as of RTC 4.0, a MS-SCCI plugin was also released allowing other IDEs (ex: Matlab, Rhapsody, ASCET, etc.) to use MS-SCCI APIs to interface with RTC/EWM. As far as I recall, it was fully compliant with MS-SCCI APIs.
An article talking about the MS-SCCI plugin: https://jazz.net/library/article/87667
So in summary, even though Visual Studio itself is capable of using MS-SCCI plugins, one would NOT use the Jazz RTC SCM MS-SCCI plugin inside Visual Studio, you would instead download the full RTC/EWM client Visual Studio plugin.