The seating plan (in Italy usually called tableau de mariage) is the board, or display, that shows each guest the table to sit at. It is not just decoration: for the caterer it is the map of the service. The same table numbering is used to organise the waiters' sections and to bring the right dish to guests with allergies or diets.
It is built starting from the groups, family, friends, colleagues, and then the special cases are placed: children, older guests, people who know few others. The final version is closed after the final guest count date set in the contract. The seating plan gives the table, the place card the seat: you do not always need both.
What to ask
Send the caterer the final list with each guest's table, name and diet, together with the room plan. Ask how it marks diets in the room. You will find the method in the guide to the seating plan and assigning seats and the one on table layout.
