I think you mean code-word clearance. This is for strictly limited-access programs, where the number of people knowing a secret is controlled. Normally, you would have a security clearance plus need-to-know. These programs put the weight much more heavily on the need-to-know side.

i.e. You may have a top-secret clearance and know that the Derkderkistani military is buying new tanks, but you don't need to know that the source of that info was from an agent in their Ministry of Finance.