CMIS aims at becoming the standard API for B2B content integration. As such, it describes an interface that a CMS must expose, as a REST or SOAP set of endpoints, to be compliant to the spec. This also means that implementing a repository is completely out of scope for CMIS itself, since it's meant to hide all implementation details behind a standard interface.
The easiest way to get started with a CMIS compliant server would be to use the Java SPIs provided by the Chemistry project, which basically implement the protocol itself and ask you to provide concrete implementations for the persistency and business logic layers. I'm not into .NET and can't advise you there.
Bottom line is, you can choose every single detail you want when implementing your CMS, and find the proper mapping to CMIS if you want to expose that API in terms of data model and operations.