GAMUser is an external object of the GeneXus Access Manager library.
The ExternalId property of the GAMUser object represents a field in the GAM User table, which has two purposes:
1. When Authentication Types are Facebook Authentication, Twitter Authentication, Google Authentication or External Web Services Authentication, Custom Authentication, GAMRemote Authentication and OAuth 2.0 Authentication the ExternalId field of the GAM User table automatically stores the ID given by the external authentication provider.
That is to say, in Facebook, Google, or Twitter Authentication Types, it stores the authentication ID given by these sites.
In External Web Services Authentication Type the value in the parameter GAMWSLoginOutUserSDT.Code returned by the external service is assigned to the ExternalId of the user. This information is processed by GAM and loaded in the GAM User table when the user logs in.
2. When using Local Authentication, the ExternalId property of GAMUser can be used freely, for example, to map the application users to the GAM users. See HowTo: Mapping Application Users to GAM Users - Using ExternalID GAMUser property.