Can someone give a detailed explanation of how the Ally/Enemy Priorities works?

[JJoules]

I have been trying to make a 9v9 (melee) map, which is split in the middle. I would like it to be that if a player spawns in one location, an enemy will spawn in the mirrored location. I have tried configuring the Ally/Enemy priorities and it seems to do absolutely nothing. The enemy will spawn next to me, they will spawn half way across the map, they will spawn on the same side (not directly next to me). .... I have looked at other maps that seem to do what im asking and have tried mimicking their configurations, which gets me nowhere.

I have also seen this post: [General] - Ally Priorities

.. honestly it doesn't help at all.

Is there already an explanation that exists that I haven't seen yet, or am I just stupid?

 

[ReforgedBike]

Scenario properties -> Ally/Enemy Priorities.
There is a checkbox "Modify Ally/Enemy priorities" which seems to do nothing when I change to "High priority" on all allies and "None" on all enemies. Should I uncheck the checkbox to make my map more optimized? In my map starting positions are pre-placed since it's a campaign coop map.
Maybe this setting is causing the trading bug where player blue can't send resources to the new colors blizzard added such as peach?