If you can save money every month on a certain subject, it’s a budget. Groceries are budget. Bills are a budget. If you travel by train occasionally, it’s not a budget.
First and foremost: a category is “incidental”. You don’t buy new furniture every month but you might want to keep track of such expenses. Or you don’t care about costs for public traffic (budget-wise) but a category would be nice.
The rule of thumb is: would you make a real life envelope for it? If yes: budget. If no: category.
Categories can be used in deposits (earning money). Budgets cannot.