Having "credentials" on ACVTS Prod means having the signed certificate referred to in Step 5 under Accessing ACVTS Prod on How to Access ACVTS. This gives the user access to the ACVTS Prod server. We use the terms "credentials", "certificate" and "access to" interchangeably in this sense.
"Authorization" refers to permission to perform validations for a particular Approved security function (i.e., algorithm) on ACVTS Prod. Authorization for a particular security function is achieved by running though certification in the Demo environment.