A commis works in a specific station, reports directly to the chef de partie, and takes care of the tools for the station.
A second commis is normally an apprentice who has just completed his studies, either at a school or an apprenticeship of two to three years in a restaurant. A second commis can be promoted to first commis based on his experience within the station where he has worked. Such a promotion might be accompanied by a slight raise in pay.
The commis is responsible for all of the tasks assigned by the chef de partie. The tasks are often the tasks that the demi-chef de partie or the chef de partie doesn’t want to do himself–but that is part of the apprenticeship!