I'm confused too. Maybe it's a hint to the human figuring out how to deploy the adapter about what kind of credentials need to be supplied. In any case, jboss ignores it.
As a human deployer, you need to know about it if you are setting up a LoginModule to supply credentials to the adapter, so you can choose one that supplies the right kind. All (both) of our jca-specific LoginModules supply PasswordCredential. So far no one seems to have tried an adapter that needs something else such as kerberos.
It appears that most people are happy with just supplying the default user/pw in the mcf config rather than dealing with setting up a login module.