Manual:External user table

external_user tracks external user accounts, if ExternalAuth is used

eu_local_id
Foreign key to user_id

eu_external_id
Some opaque identifier provided by the external database

Schema
++--+--+-+-+---+ ++--+--+-+-+---+ ++--+--+-+-+---+
 * Field         | Type             | Null | Key | Default | Extra |
 * eu_local_id   | int(10) unsigned | NO   | PRI | NULL    |       |
 * eu_external_id | varbinary(255)  | NO   | UNI | NULL    |       |