The remote user ID for a user is the value from the LDAP server (e.g. Active Directory), which ensures that the COYO user and the user account from the LDAP server are connected.
You define what value is stored there in the configuration of the user directory in COYO. There, an attribute under "User" is defined as "User ID".
In the Active Directory, this ID can be checked for the example shown above e.g. in the Attribute Editor (or via PowerShell) for a user.
If these values are different in COYO and on the LDAP server, the user is orphaned. And so can no longer log in. By correcting this value, local users can also be turned into remote users.