While typing the question I had an epiphany and found the answer.
The error in my question is that I want to do a GetCapabilities request and specify the content, i.e. identifier/coverage.
It is not needed to specify any other keywords in the GetCapabilities request.
If I want to do a request of a specific layer/content/identifier/coverage (many different terms), a GetCoverage request is needed.
With wcs version 1.1.0 a GetCoverage request can used. With the key word IDENTIFIER, I can specify the content from which I want coverage.
Furthermore, the following keywords are mandatory for a wcs version 1.1.0 GetCoverage request: VERSION, REQUEST, IDENTIFIER,BOUNDINGBOX, FORMAT.
But I am just touching the tip of the iceberg, for a fully detailed explenation, see: http://mapserver.org/ogc/wcs_server.html
No there isn't because GetCapabilities requests don't need any other keywords. If you want to specify the content of the WCS request, use for example, the GetCoverage request.