You’ll usually find commis chefs in large operations, like hotels, resorts, big restaurants, those kinds of places. In American kitchens they’re usually called prep cooks.
A commis chef’s chef duties could be a combination of things including: receiving and storing food deliveries, gathering and prepping ingredients for the line cooks to use, setting up line stations, cleaning, those types of things.
A normal shift may be: receiving a fish delivery, checking the fish for freshness and weighing it to ensure the order was filled correctly. Filleting the fish for service, placing the filets into pans and placing them on the line with bags of ice on top. Giving the fish bones to the saucier for stock and then cutting and blanching vegetables for the line, again giving the scraps to the saucier for stock. After the fish and vegetables are prepped, they might cut meats into portions and distribute the meats to the line stations, scraps again going to the saucier. Just before service the commis would prepare starches, like rice, mashed potatoes, whatever’s on the menu. If the restaurant cuts and blanches there on French fries (an arduous job), the commis will be doing that too. During service they might be assisting a station with cooking or plating, restocking the station between seatings, cleaning up messes, or doing whatever else needs to be done. During service they are doing the small tasks that the line cooks can not do, since the line cooks are busy preparing food. Once it’s all over they may be taking out the trash, taking dirty pans to the dish room, cleaning, wrapping and storing food, that kind of stuff.