Fallback ID methods
If other visitor ID methods fail, Adobe sets a fallback cookie or uses a combination of IP address and user agent to identify the visitor.
Fallback Visitor Identification Method
With this update, if the standard s_vi cookie is unavailable, a fallback cookie is created on the domain of the website with a randomly generated unique ID. This cookie, named s_fid, is set with a 2 year expiration and is used as the fallback identification method going forward. This change should result in increased accuracy in visit and visitor counts in situations where the primary cookie ( AMCV_ or s_vi) cannot be set.
Visits total includes all visitors that are identified by the s_vi cookie or by using a fallback method.
IP Address, User Agent, Gateway IP Address
. If the AMCV_ or s_vi and the s_fid cookies cannot be set, visitors are identified using a combination of IP address and User Agent.