User Idnumbers: ELIS uses an idnumber to keep track of users - this means that the user's data is always attached to the same person, regardless if their name, email, or other information changes. This also means that in order for users loaded from Moodle to import correctly into ELIS, they must have an idnumber set in their user profile. By default, if users are loaded into an ELIS enabled Moodle site without idnumbers, ELIS will copy their username and use that for their idnumber. Once the idnumber is set, it can't be changed - if it is changed, ELIS will create a new user for that idnumber.
The key to this master data management methodology is simple: idnumbers don't change.
If a site admin needs to have users in Moodle who are never brought into ELIS - they can turn off automatica idnumber assignment in the Configuration settings for their site. In this case, admins will have to generate idnumbers for the users they do want in ELIS or provide idnumbers as part of the user loading process.