Are there specs around how User IDs are assigned?

[timfin]

I'm looking for a better understanding of the options around the user ID values Canvas assigns to accounts.  The end goal is to have a distinct user ID that can be assigned per person and follow them throughout their entire lifecycle in relation to our program, whether they are an instructor, learner or both. And can also be passed to other systems for tracking.  If there's documentation out there that explains all this then that'd be another place to start.

• They appear to be sequentially assigned in order of creation. Is that accurate?
• Are the numbers ever re-used per client?
• If an account is merged, is a new value assigned or is there a sequence to choosing which account to merge with?  Like always choose the lowest or older sequence to assign?
• Do we as the client have any input or potential to have any input to how the values are assigned? instead of having the value be arbitrarily sequential, can there be some logic behind how they're assigned?
• Is there another custom field at the account level that could be more easily controlled by the client to assign a more robust identifier?  And formulaically applied by Canvas at the point an account is created?

 @samkra ‌