In the British army the most senior officers of any particular rank up to lieutenant-colonel (lieutenants, captains, major etc) served in the 1st battalion of an infantry regiment, while the junior officers served in the second battalion (where there were two battalions as was commonly the case). This meant that there was a good deal of movement of officers between battalions, which could cause problems when the first battalion was on a distant station.
In the French army infantry regiments had a greater number of battalions, but all except a weak depot battalion generally - but not always - served together.
Did French officers shift between these battalions as they gained seniority, or did they remain in the one battalion at least until they were promoted up a rank?
Also, what was the basis of promotion for regimental officers in the French army? Merit? Seniority? Did most officers spend their whole career in the one regiment?