How is the user ID generated?

In case there is no user ID is sent, it will be generated by us based on the below logics in different scenarios:

1. 'Username' and 'Email address' is sent, it will be generated based on these 2 data points
2. Only 'Username' is sent, user ID will be generated based on the 'Username' field
3. Only 'Email address' is sent, user ID will be generated based on the 'Email address' field
4. Neither 'Username' nor 'Email address' is sent, random string will be generated by us for user ID (without it being a duplicate)

Please note that the user IDs will be different in these 4 scenarios, even if the user is the same. Therefore I would advise to include the user IDs in all the requests if possible.