I think it may depend on the map you are using. But from my expirence with MA (have made and tested quite a few missions) the placement doesn't seem to do anything. 9/10 times the bosses/allies/glowies have just shown up in random places. So far there seems to be no way of fixing this.
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On open maps, yes, it doesn't do anything at all.
Indoors it works quite reliably as long as you don't ask too much of it. That is, there must be spawn point for either "boss" or "any detail", enough space to actually spawn it, and enough spawn points nearby for ambushes, etc. Also some maps are bugged or have pretty unusual definitions of what "front" and "back" mean (esp. warehouses).
Question, is it the actual Boss itself, with the right texts and everything, or is it just a Boss class enemy?
If the spawn system needs a Boss and there is no other Bosses availible it will use the only Boss in the group. There are two ways around this, either place your Boss in another enemy group, or up the rank of your boss to EB level.
The map is a tech abandoned set, medium i believe.
And the boss is a boss class enemy, with text and the like. It just appears at the front, but ONLy when there is more than 2 people in the team.
Otherwise, the map seems to define the spawn points for my events pretty well.
Does the boss appear at the front in addition to appearing at the back?
If it does, then it is spawning as a regular boss because it is in the enemy group assigned to the map.
I really should do something about this signature.
AH Thanks praf, that is a possibility.
as for the 'in addition part' i can't verify this 
The mission is sort of difficult, so i've never really made it to the bottom of the mission.
Yea min/lieut/boss classes will spawn in normal mobs eb/av wont. It would make sense that higher than 3 people makes bosses spawn in the mission, i would suggest making an EB. which might make it slightly tougher encounter but at least if you then removed the "boss" rank character it wouldn't spawn in the normal mobs. 
Easier just to put it in it's own enemy group, then it can stay as a boss without any chance of spawning by mistake.
I really should do something about this signature.
Hi, my mission is difficult when alone, which is what i'm aiming for.
My problem begins when i test my mission in a team. My settings have the boss set to the 'back' of the map. However when a team with me enters it (was only 3 members) the boss always spawns right at the front. It's annoying, and any help is appreciated on how to fix this.