“grey” content

In an email from Sean Parker to Daniel Ek, in 2009, this sentence jumped out:

4. There are vast repositories of “grey” content on people’s computers across the world. This content is often forgotten by the labels and publishers. The fact that you have a client and P2P capability will allow you to someday unlock all of this content…this goes way beyond the master music catalog that is licensed to you by the labels. A big part of Napster was the joy of discovering various remixes, live recordings and other obscurities that copyright owners have literally lost track of.