From what I understand it was building types and timers. In the game data there are defunct money buildings like Mini-Mart and Adult Book Store (or something like that) and all of the retired buildings have five minute timers for every upgrade. Someone mentioned that, at the very beginning, all upgrades were indeed five minutes.
Then defense buildings I guess. I'm pretty sure they meant to make 5 of each available rather than 50. That's how it is in Modern War.