I have been reading a lot of stuff on understanding how visitor identification happens in Experience Cloud solutions - specifically Adobe Analytics. I am clear with the back end request calls between ID service and Data Collection server for creating Marketing cloud visitor ID within or across domain. However, while going through Why the New Adobe Marketing Cloud ID Service Should be on Your Radar | Adobe Blog I noticed the importance of having s_vi cookie.
My doubt is what is the significance of having s_vi cookie if we already have Marketing Cloud Visitor Id to identify unique visitors? Moreover, how Analytics ID (AID) fits in this whole visitor identification and why s_vi cookie is the one which is checked for the first and then AMCV cookie (highlighted in the texts above)? I thought AMCV cookie was enough for visitor identification both within domain and cross domain.
The s_vi cookie is a legacy cookie. As you pointed out, the MCID service checks for an existing AMCV cookie first. This is to see if the visitor is a return visitor. If the AMCV cookie is found, this means the visitor has been to this site at least once since the MCID was deployed. If no AMCV cookie is found, we generate one. As part of generating the AMCV cookie, we need to check for a legacy cookie and id to make sure we don't cliff your visitor.
A few videos I made that I'd recommend watching are listed below: