Each spawning point has a permitted spawning list, which determines what type of enemies it might create. The exact decision about monster type is determined by the level and whether or not the enemy has any Tact Points. Depending on whether they do or not, the enemy runs one of two simulated economies to determine spawn picks. Spawn locations are not equivalent to each other; some are deadlier than others. The spawning itself takes place in two waves; with only those spawned in wave one capable of moving and attacking in the turn they appeared.
The decision whether or not to spawn from a spawn point is not affected by how many enemies are on the map unless the enemy squad cap (not shown to player) has been reached, as it is based on Difficulty and story related conditions. Story conditions act as an on/off switch for spawn points, such as reaching an area to trigger an ambush. The other ways to alter spawning rates are to destroy any spawning towers present and to cover a spawning point with flames (this prevents spawning occurring there for the duration of the flame). Overwatch will only inhibit the movement of freshly spawned enemies.