Certain operating rules allow trains to give up track behind them to a following train, and with NORAC/DCS we can call clear of a milepost, which then allows the dispatcher to issue a Line 2 to a following train up to that milepost.
Before issuing a Form D authority in non-signaled DCS territory, the Dispatcher must determine that the track to be used is clear outside yard limits.
Overlapping Form D authorities for opposing movements must not be issued.
The second sentence would imply that you can give multiple Form Ds as long as the trains are operating in the same direction, but the first sentence directly contradicts that by saying the track must be determined to be clear, hence the need for the first train to call clear of a milepost before the dispatcher can give a following train a Form D up to the milepost called clear of.