All classification concerns access control. What is not always recognized is that in addition to any clearance to whatever level, there must be a "need to know" established by authorized parties. Just because you are cleared for "Top Secret" does not give you permission to access TS beyond your established need to know.

The SCI category is a further level of control, and can rightly be regarded as a higher classification level. It can cover information beyond intelligence sources and methods. (Been there, done that.) And there can be compartments within compartments.

There are other categories, e.g., Critical Nuclear Weapon Design Information (CNWDI) and NATO Confidential, Secret, and Cosmic Top Secret.