It's not very clear in the tutorial how it works exactly. Lemme try to break it down for ya:
You have a unit cap at the bottom left of your screen. As the match progresses, this cap slowly opens up to allow you to have 6 units on the field. Losing one of these units frees up the slot and allows a replacement to be built instantly. This prevents players from spamming low cost units to fill the map, or overwhelming their opponent. It also helps players make a come back when they are behind by now allowing their opponent to out-produce them.
So taking all that in mind, here are a few key strategies:
Harvesters take up a unit cap! 2 harvesters take up 2! This means you only have 4 unit slots left to play with for the most part until near the end of the game where you get the last one. So going double harvester when your opponent doesn't means you get an eco advantage, but are at a unit DIS-advantage!
If the enemy has a low health unit that doesn't pose a threat to your army, try to leave it alone and not kill it. Let it hit your base or harvester. That is now 1 less unit they can produce giving you an advantage over them as long as it remains alive.
On the flip side, don't try to preserve almost dead units. If something is just barely alive, it won't do you much good, and is almost always better off getting sent to their honorable end and having a fresh unit replace them. The only time this strategy isn't valid is when the nuke is going critical and you need every able body to rush the pads.
Hope this helps!
