Not all unions use DOH...I have posted this before.
Within some unions seniority is not a factor, so it does not even matter, others use methods commenserate to their particular industry in order to assign jobs, and positions are determined by other factors as well.
So, you are wrong in your "virtually all other unions" statement. Actually very wrong.
This interests me Jake, as I'm not against gaining more knowledge of any given subject. What particular unions do you have in mind in your post here?