This is one thing that has always stumped me about Marketing Channels.
Marketing Channels are applied with every hit. And channels like Paid Search, Display, etc. are usually configured where the Query Parameter is set with some value. So I understand how the first hit of a visit would be able to be categorised under one of these Marketing Channels.
But what about the 2nd, 3rd, 4th...nth hits? These hits don't have the Query Parameter any more, so how would AA know that they should "belong" to the previously set Marketing Channel?
As @Andrey_Osadchuk noted, the value persists much like an eVar, but with two (2) allocation methods rather than one (1): First Touch and Last Touch.
The broader answer to this question is... It depends on how you've configured Marketing Channel Processing Rules for your report suite.
For every hit, Adobe runs the call through your Marketing Channel Processing Rules. If any one of the rules finds a match, the new channel is set, and the Last Touch channel is updated. However, if none of the rules match (no query parameters or other relevant values), no change is made and the existing Last Touch channel persists.
Marketing channels variables persist and are also checked for every hit. Once they're set on first hit of the visit, that value continues to persist unless overwritten by a new value on any further hit.
Hit 1 : A visit starts with Paid search. MC = Paid
Hit2 : Next page of web site , MC = Paid // since value is persisting
~~Now user goes to social media and then clicks on a link to land on the same website. It'll be considered as same visitor.~~
Hit 3 : MC = Social // channel is overwritten with new value.
Every hit checks for a Marketing channel processing rule. If the hit matches the condition of the rule Last Touch would get updated. If the hit does not meet any processing rule old value set for that visitor will persist.
If the visitor is new, does not have First and lat touch set and hit does not meet the criteria of any rule that hit will go under None.