We might need more details to understand your query here. If it is related to how unique visitors are counted, then this the order:
The method for identifying unique visitors among multiple contexts includes a prioritized sequence to ensure accuracy in this determination. The following table shows this prioritized sequence:
|1||vid s.visitorID||0||s.visitorID is set.|
|2||aid s_vi cookie||3||Visitor had an existing s_vi cookie before you deployed the Visitor ID service, or you have a Visitor ID grace period configured.|
|3||mid AMCV_ cookie set by Identity Service||5||Visitor’s browser accepts cookies (first-party), and the Identity Service is deployed.|
|5||HTTP Mobile Subscriber header||2||Device is recognized as a mobile device.|
|6||IP Address, User Agent, Gateway IP Address||1||Visitor’s browser does not accept cookies.|
For information on how unique visitors are reported, see Unique Visitors in Analytics.
You might also want to look at this documentation to see what each query parameter/property in the payload send to adobe analytics correspond to:
Also anything between c.a. and .a.c are adobe reserved variables that should be automatically mapped to some reports may they be mobile reports, video reports and so on.
The 3rd Id probably denoted something about the type of activity. Don't think it was the device ID/name. But it is largely ignored and not used anymore.
This doc on A4T reporting also tells us to ignore it 🙂