All forms of music. Classical, Rock, Reggae, World etc.
These are recordings that were made available to the general public.
Other Commercial Recordings
Commercial pressings that were never intended for general sale. These include Demonstration and Promotional material.
Commercial style formats (e.g. disc and CD) that were made for reasons that do not fit into the above categories.
Studio Masters (and session tapes)
All studios keep a huge archive of sessions and master tapes. Many of these contain material never heard by the general public.
With the advent of inexpensive tape recorders and later cassette, DAT and MiniDisc many performances are recorded that would otherwise have been lost forever. While most of these would probably infringe copyright, they are a wonderful source of reference.