Should only be used in case of a hybrid CMIS Scenario:
OauthToken AND open Documents from SAP in an external browser.
The tokenseckey authentication type is a combination of token authentication and SAP seckey. For SAP users, that need the Document opened in an external application (like a Browser), SAP created this process.
The procedure ensure, that the machine Token (configured in SAP) is not exposed to a Browser where it could easily extract and access other resources on the CMIS Server for that repository.
See following subsites:
OpenID configuration - tokenseckey (tC)