Do you mean the space directly surrounding the aft grand staircase? That is marked "restaurant reception room" on one plan. It would be just that -- an area for people to wait. I can't recall whether drinks were served there. The restaurant was quite popular and often busy, so people would need a place to meet up with their dining companions before their reserved table became available.
If your plans show promenades next to the a la carte restaurant, they are plans of Olympic. Titanic's restaurant was larger than Olympic's, because it had proved more popular than expected. Olympic's restaurant originally had a bar, but it was removed to provide some extra space, and Titanic had no bar. Titanic also had a larger galley/pantry for the restaurant, and of course the Cafe Parisien.