Early versions of ISO 9001 were very particular in their language that indicated different levels of documentation. ISO 9001:2000 is less specific. If you see value in classifying documents according to “levels,” then by all means do it. There’s no requirement in ISO 9001 that you do this, however.
There are other standards that specifically require that you define the structure (i.e., levels) of the documentation system. That’s the case with The Coca-Cola Quality System (Evolution 3), which requires that the document control procedure include, “a. Structure and numbering of the documentation system.”