The Signing-on-Sheets are not as clear as they could be, but the term "Assistant" seems to be used as in Assistant Pantryman Steward; Assistant Boots Steward and with some of the Public Room/Deck Stewards as well as two Assistant Second Stewards; but not as Noel notes with the Stewards, who are as far as I can tell listed as "Stewards" and Bedroom or Bath [varietally Bathroom] Stewards. Even the word "Saloon" does not seem to be there. - I note that both Saloon Steward and Assistant Salon Steward appear in the Crew List on this web-site. I have no idea where the "Assistant" part comes from, but I guess "Saloon" was added to make the duties clearer. - That was done with the Engine Room crew. So rather than just having Fireman they show as Fireman/Stoker.
However, the Restaurant had Waiters and Assistant Waiters.