What it means is that you can't be released to RAP from Standby.

It means you can be released by contacting CS.
You can call at noon if you want - they will often release you if you are not the most junior pilot after S2 shows up, and the day isn't a total clusterf. But they can't say "ok you are good to go" and leave you on RAP.

The whole premise of 117 and duty regs is, that your duty time is preassigned and your rest is prospective. If you are assigned duty from x to y, there's nothing that says you need to be "released" from that duty. You are released when clock hits y, unless someone calls you and tells you otherwise.
The contract says you may be assigned to be on ready reserve for up to eight hours. After 8 hours, your day is over.

12.F.3 does not mean you have to call to get released.

Someone who knows the history of the contract might know better why they worded it that way, but since it's never used nor abused, I don't think anyone really cares.
But there's no reasonable interpretation that it says you HAVE to call to get released, it just doesn't read that way. "Remainder of the day" - your day ends when your duty ends.