My understanding is as follows:

Airlines load flights into their computer systems about 11 months in advance. However, you can reserve a flight more than 11 months in advance. Such a flight is outside system range. The booking is recorded outside the main reservation system. It will be transfered into that system after the flights in question are eventually loaded.

If you book a round trip whose return (inbound) flight is more than 11 months in the future, so it's outside system range, you get one free date change to that flight.