This is something most Adobe users do not know nor expect nor does it make much sense: Marketing channels have a rolling 30-day expiration, whether a channel was touched or not. Tracking codes have an expiration based on when the variable was defined.
"Expiration differences: Marketing channels have a rolling 30-day expiration, whether a channel was touched or not. Tracking codes have an expiration based on when the variable was defined. For example:
A user arrives on your site via the tracking code promotion3, then leaves.
They come back three weeks later without a tracking code or marketing channel, and exit the site again.
Another three weeks later, they come back without a tracking code or marketing channel, then make a purchase.
The user ultimately made a purchase beyond 30 days but was never inactive for more than 30 days. Therefore,
Marketing channels attribute revenue to the campaigns channel, since the visitor engagement is reset each time they return to your site.
The tracking code report would attribute revenue to "None," since the purchase event occurred beyond the variable's expiration."
See this user for example: He came through Channel "Paid Search G. Shopping" on Jun 3 (M Channel Instances was set). That was also his last Marketing Channel Instance.
However when we look at this user's data on Nov 3, it is STILL attributed to Paid Search Shopping even though 30 days since his last Channel Instance have long passed!
This is - imho - absolutely not what a Marketer would expect. It is also unnecessarily inconsistent compared to eVars logic (tracking codes etc.). It is furthermore not documented on the Marketing Channels Expiration page, where just some fuzzy sentence concerning "First Touch Channels" is found.
In Marketing, you usually want to usually acquire someone once and then you want to see how many of those who were acquired return by themselves (e.g. via "Direct"). Your hope is that you do not have to re-acquire them again and again through Paid Channels. In the example shown above, Marketing will draw the false conclusion that they are re-acquiring this customer over and over again through Google Shopping, which is NOT the case. A user who has returned directly for almost half a year is still attributed to Google Shopping just because he never had more than 30 days between visits...
Adobe needs to change this logic to be in line with eVars expiration. I bet most clients do not expect Marketing Channels to work like that.
Even better would be a toggle option in the Expiration Settings like 'Expire xx days after most recent marketing channel instance' or 'expire xx days of visitor inactivity'.