How is the table app_user being populated?

[Vinay Srinivasan]

I am facing issues when we are calling the following API

https://jira.URL.com/rest/api/2/project/xxx/role/10002

 error message : {
"errorMessages": [
"We can't find 'xxxxxxxxxxxxx.com' in any accessible user directory. Check they're still active and available, and try again."
],
"errors": {}
}

On checking I found that the user_key and lower_user_name columns are not having the same value.

ie  user_key will have first part of the email id

    lower_user_name will have full email id

I want to know how the data is being populated in this table and where all the user_key values are being used