Master keys consist of 5 bytes of random data when an export cipher is used, and 16 bytes of random data when a domestic cipher is used.
When the CSP is domestic, it is not aware of the cipher suite when CPGenKey is called, thus the full 16 bytes should always be generated. If the cipher is an export cipher, 11 bytes of the master key can be overwritten by clear key data (within CPSetKeyParam). When the CSP is exportable, then it can only generate 5 bytes of data and will expect clear key data to be set with CPSetKeyParam.