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 GAM Authentication Types are GAM Facebook Authentication Type, GAM Twitter Authentication Type, GAM Google Authentication Type or GAM External Web Services Authentication Type, GAM Custom Authentication Type, GAM Remote Authentication Type and GAM Oauth 2.0 Authentication Type 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 GAM 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 GAM Local Authentication Type, 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.