The "waiters* " were saloon stewards.
The "attendants" were bedroom stewards.
The deck officers, engineers, seamen, firemen, trimmers and greasers all had their own individual mess rooms.
Yet, the stewards and galley crew don't appear to have had their own assigned mess rooms.
There has been a suggestion (and probably a correct one) made here on ET in the past that the stewards and galley crew simply ate their meals in the galleys and pantry when they could get the chance (for they would be kept very busy of course) and maybe also had a snack whilst lying in their bunks.
Regrettably, only rarely were the surviving crew (regardless of which department they were in) asked about what their own accommodation and victuals were like sadly, a real missed opportunity. No-one was really interested (I'm interested though !) in what it was like for a first class bedroom steward when he was off duty, they were more interested in the people he would have served.
It's just my opinion, others may disagree but I think Violet Jessop's memoirs are the best "window" into what life was like for an ordinary member of the victualling crew.
*Only the À la Carte Restaurant had "waiters", and they were not strictly employees of the White Star Line.