Every part in the game including cubes, weapons, modules and cosmetics has a Robot Ranking stat; you'll be able to see a part's ranking stat by hovering over it in your inventory. When constructing/editing a Robot, you'll see both the Robot's current Tier as well as a running Robot Ranking total. Adding more parts or changing parts for ones with a higher ranking stat will increase the ranking stat. Removing parts or changing them for parts with a lower ranking stat will decrease your Robot's ranking. Once a Robot ranking moves above or below a threshold, the Robot will change up/down a Tier. This system allows you to fine-tune your Robot to fit a desired Tier.