A 'routine arrangement' is a situation that is considered everyday, regular or scheduled.
A weekly (or daily) appointment or meeting at the same time and/or place and/or with the same participants.
An agreement to do something on a regular scheduled basis... Feeding your cat in the morning is a morning routine. Or meeting your old friend on vacation every year would also be a routine arrangement. Also going to your parents for dinner every Sunday night.
The way tables or seating my be organized at an event could be considered routine in the way they are arranged.