Reconcile is a way of make sure that a user account exists and has the correct data. The remote system is treated as the system of record, but we want a record in our local system as well.
It is a two phase process:
If the user doesn’t exist in FusionAuth (i.e. the OAuth server), they are created
The data from Facebook (or any third party system) is merged with the data in FusionAuth and the user is updated
Step 1 is optional and configurable in FusionAuth. We call it Create Registration in the application configuration. If the user already exists, then step 1 is skipped.