I would want to ask for this very helpful feature in the game. Why is it helpful? When the player has little time to wait for a certain building to be upgraded, he can put the building to be upgraded on the background on queue. Let's say you have a building that would be upgraded within the next hour but you have to leave the game before the next 20 minutes are up. This is where the queue comes in, you can only passively upgrade a maximum of 5 units/structures per base (as to prevent exploitation), like lets say an accumulator, followed by a venom, then a defensive missile squad, then the offensive one and one silo in order. This becomes useful for certain buildings that tend to be raised above the levels of other buildings, like the accumulator and silos.