Files
Producers live in project files, and whether the project file moves is its own question. Most collaborations exchange a trackout instead: every element as its own full-length WAV from bar 1, at the project sample rate, with the BPM and key in the names. Ask for the trackout rather than "stems", which can mean grouped submixes. MIDI files move when the other producer needs to change the notes, and the project file itself only moves in a true co-production or a ghost production handoff, with the plugins listed and the samples consolidated. Remixers get the trackout or a parts pack plus the acapella. Mastering engineers get a pre-master WAV with headroom and no limiter on the master bus.