Any tape used in the production of a master tape. For example, if a master tape is an equalized (or otherwise processed) copy of an earlier tape, the earlier tape is called the sub-master.
"Over the years I have treasured all the things in my life that mean the most to me. One of these was my "Oldies-But-Goodies" record collection, but as the years went by they became scratched and worn. I wanted to keep them, but I wouldn't play them any more. Then I learned about CVC Productions. I was told they could transfer my old records ..."