The amount of Files in your track. This will include: Vocal takes and Beats Files/Stems.
Stems are a group of files, typically grouped as: Drums & Bass, All Instruments & All Vocals.
You can upload your Files here (www.wetransfer.com) or any other large file sharing platform.
A Mix Revision gives you the option to have your Mix revised. Meaning, any tweaks you want, will be done and sent over as a new revised Mix edit.