Usually, the scenario goad as below to have a BC user created:

> A user mail is setup on M365 admin center, and accordingly this reflects on Entra

> Such user gets assigned the needful BC licensing

> On BC UI you click on sync from M365 then a new user account is created on BC for such user

What Sometime happen is:

> User leaves the company so his entra account is soft/hard deleted

> User returns to the company

> We need to recreate his BC account by relinking his old BC user name to his new M365 mail/account, or by being able to have his old BC user deleted to have a fresh new one with the same exact name, the main reason is to have the proper name for this user again on BC.

Since BC does not support such re-linking procedure, and does not support deleting the user from its UI, the admin is stuck with only one option which is to create a fictional user name for the user (not in his same previous name), because in this case, business central puts a 1 next to it because its a duplicate