Situation: your album is done, mixed and mastered, finished. How are you going to preserve your sessions for the future? Probably never even thought about that, did you?
I've seen situations where people need to access their session files five, seven years in the future. The thing is, these digital recording sessions that we use over time- the plugins and software compatibility changes, and you could be left with a session that you can't access anymore or that doesn't sound the way it sounded when it was mixed.
Check out this list, these are the WAV file stems you want to end up with at the end of any project. These stem groupings will allow you to later make any show mixes you need, remixes, and alternate versions for sync licensing opportunities needing only access to any DAW.
Final stem WAV files to request:
Lead vocals
BGVs
SFX
Horns
Strings
Keys
Guitars
Percussion
Drums
Bass
Click
Guide
Now, these will take hours for your engineer to make. You're likely going to get charged extra for this, but I think it's worth it if you're seriously trying to monetize and promote your music going forward.