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[GM1361]

Level Grade	Dynamic Brake Available	Slack Bunched
Ascending Grade	Dynamic Brake Not Available	Slack Stretched
Descending Grade

 What is meant by "dynamic brake available or not available?

B. Stopping—Scenario 2

When stopping under these conditions:
1. If in power, gradually reduce the throttle to IDLE.
2. Wait for the slack to adjust.
3. At a sufficient distance from the stop, make a minimum brake
pipe reduction and actuate.
4. Make further split reduction(s) as needed and actuate.
5. As the train comes to a stop, make a final brake pipe reduction
and allow the locomotive brakes to apply.

[jeffhergert]

I think when they use available or not available, they are meaning that the engineer can or can't use the dynamic brakes under their instructions. 

Going uphill in power the slack is stretched out.  Using the dynamics would start pushing slack back into the train.  If going slow enough, you might be able to have the rear end roll back, even while moving, enough to snap a knuckle or draw bar.  When stopped with the slack bunched up on an ascending grade, the rear end could roll back when the air brakes are released when ready to depart, again possibly snapping a knuckle or draw bar.    

Jeff

[GM1361]

Can it mean the train is equipped or not equipped with dynamic brake? The following sentence from the same rule book may help:

NOTE: If the dynamic brake is unavailable or ineffective,
use the independent brake to maintain a slack-stretched
condition.