A userID is a number assigned to every 4D user listed under the Toolbox > Users tab. For the default users, the userID assigned are 1 to "Designer" and 2 to "Administrator" and this rule will remain true for both binary and project mode. The difference in userID will matter depending on whether the database is in binary or project.

UserID in binary mode:
UserIDs are based on when a user was created and will keep the same userID unless converted to project mode. Take the example below:

Designer - 1
Administrator - 2
UserA - 3
UserB - 4

Upon deleting UserA, UserB will keep userID 4 as so:

Designer - 1
Administrator - 2
UserB - 4

UserID in project mode:
UserIDs are based on when a user was created and will update itself upon another user's deletion. Take the example below:

Designer - 1
Administrator - 2
UserA - 3
UserB - 4

Upon deleting UserA, UserB will reset its order number to the 3rd user:

Designer - 1
Administrator - 2
UserB - 3

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Keep this differences in mind in case there are any calls to Validate password against a userID.