Caterer falls in Class 43, restaurants & hotels. Everything below is read from the USPTO's own list of accepted descriptions, not from an opinion about where it belongs.
Searching the USPTO Trademark ID Manual for caterer returns 91 accepted descriptions, and all of them sit in Class 43. The manual is the list of identifications the office has already approved, so wording taken from it is pre-approved and settles the class at the same time.
The class follows from the identification you write, so "what class am I in" is really "what exactly do you sell". How the class system works covers the rest.
You serve food or drinks or provide lodging — a restaurant, cafe, bar, or hotel.
91 accepted descriptions in the manual mention caterer. For example:
Caterer is a service, so it sits in Classes 35 to 45. Software that delivers a service is often Class 9 or Class 42 as well.
Restaurants, catering and lodging: restaurant services, catering and hotel accommodation are here. The food itself is a good in its own class, so bread is Class 30 and cheese is Class 29.
The class decides what a registration covers; it does not decide whether the name is available. Relatedness is judged on the identifications, so a conflict can sit in a class you never looked at: the same name in a different industry.
Check whether your name is clear for caterer Searches the live USPTO register, including sound-alike and meaning-alike names. First search $2, then $5.Caterer sits in Class 43, restaurants & hotels. That is where the USPTO's own ID Manual puts it: 91 accepted descriptions mentioning caterer fall in Class 43.
You file in every class you actually use the mark in, and the USPTO charges its fee per class. Selling a product and also running an online store for it is two classes, not one. Filing in classes you do not use is worse than filing in too few: an application has to be supported by real use or a genuine intent to use.
No. The class decides what a registration covers. Availability turns on whether someone already holds a similar mark for related goods, and relatedness is judged on what the identifications say rather than on the class number, so marks in different classes can still conflict.
Not what you sell? Search the class finder for your own wording, or browse all 45 classes.