A better way to look at it is this. ARCOS has two steps:

Step 1) "Hey Mr./Ms. Pilot. Here are some trip(s). Are you interested, and if so, which one's do you prefer?". The answer can be none, thus, the whole thing is a proffer.
If you select auto-accept, you are saying "I will look at any rotation that you proffer, but you don't need to call me for this step. Don't call me unless I'm the person the trip would go to for sure".

Step 2) "Hey Mr./Ms. Pilot, you are the successful bidder for a/the rotation from step one, the trip is yours. Do you really want it?". Again, the answer can be no for this step as well, so it's also a proffer.
If you select auto-acknowledge, you are saying "I will accept whatever rotation materializes for this step, so you don't have to call". The system will call you anyway if inside 12 hours.

If you select auto-accept, but not auto-acknowledge, the system won't call you unless you're the successful bidder. You then get 12 minutes to decide if you want to do it or not.

Both steps are proffers. That's the big takeaway.