Why is my contact custom field not updated through the pass through authentication (PTA) process?

Environment:

Pass Through Authentication (PTA), Version February 2013 and newer

Issue:

I am passing a contact custom field through in the PTA string. However, the contact created/updated by the PTA process does not have this field set.

Resolution:

In order for contact custom fields to be updated when passed in the PTA string, end-user visibility must be set to "Read/Write" for that contact custom field. The end-user visibility setting is managed from the Custom Fields editor (Configuration > Database > Custom Fields) under the "Interface Visibility" section.