My question is asked largely with WW2 in mind, though I am interested in the protocol in general. It would make sense to me if the highest ranking officer would be designated as Senior Officer, but in a camp with officers from different branches, or where there are multiple officers of equivalent rank, how was seniority decided?
Seniority of rank is a standard protocol. It has to be because the question of who is in command is extremely important to militaries and comes up regularly. Across the branches of a particular country's military, equivalent ranks across branches are clearly identified. Where more than one person holds the same or equivalent rank, seniority is by time in rank, ie promotion date, unless an officer has an earlier declared seniority, or has lost seniority as a result of disciplinary action.