Order of Operations for Analytics IDs
After you deploy the visitor ID service, there are 5 ways a visitor can be identified in Analytics.
In many scenarios you might see 2 or 3 different IDs on a call, but Analytics will use the first ID present from that list as the official Experience Cloud ID. For example, if you are setting a custom visitor ID (included in the vid query parameter), that ID will be used before other IDs that might be present on that same hit. See Setting Analytics and Experience Cloud IDs for more information.
A browser doesn't accept third-party cookies and the Analytics tracking server is set up as a third-party tracking server.
Note: The fid is a legacy identifier and is not used if you've implemented the ID service on your site. In this case, the fid is not needed because the first-party, AMCV cookie makes it obsolete. It has been retained to support legacy code and for historic reasons.