This service is part of iCloud Music Library, which is a feature of both MUSIC and iTunes Match. It uploads songs that did not match against the iTunes music collection, and does not count towards your iCloud storage allowance.
iCloud Music Library as a whole is a broader service that encompasses the following:
- Purchased tracks (previously called iTunes in the Cloud)
- Tracks matched or uploaded with iTunes Match
- Tracks matched or uploaded with Apple Music
- Streaming tracks that you’ve added to My Music, if you have an Apple Music subscription
When the “music locker” functionality launched originally with iTunes Match, the limit was 25,000 songs. However starting in December 2015, this has been increased to 100,000 songs.
Note that some songs may be ineligible to be either Matched or Uploaded. This includes:
- Songs with a bitrate of 96 kbps or lower
- Some songs encoded in 24-bit
- Songs larger than 200 MB
- DRM-protected songs, and songs your computer is otherwise not authorized to play
- Digital booklets and other non-song items in your iTunes Music Library
- Music videos that weren’t purchased on the same iTunes account
Regarding the first two, you can re-encode them in 16-bit, 128 kbps, and you might get a 256 kbps Matched version out of the deal.