Crew resources has stated in the past that they can under fill vacancies (remember when they posted 744 slots that they never filled due to aircraft changes after the AE came out), but they can't fill more than posted (and backfill).

It's a fairly good gauge to look at open spots after an AE IMO. They'll still probably send the most folks to the more junior positions, but it gives you an idea where they need bodies. The NYC narrow bodies are revolving doors of the system so they always have a ton of spots posted (more than they really probably need).