Tiers are separate Tank Classes, organized with the characteristics of their Cannon(s).
Tiers are represented in the Class Tree, shown on the picture to the right, as a series of concentric rings containing every Class in the game. It can be displayed in-game by holding the Y key. In the direct center of the Class Tree is the starting tank; or Tier 1, or Level 1. The first ring contains Tier 2 (Level 15) tanks, the second ring contains Tier 3 (Level 30) tanks, and the outermost ring contains Tier 4 (Level 45) tanks. Areas shaded black on the tree indicate an unavailable Upgrade in that tier.
Interestingly, there is a recurring pattern of colors in the upgrade selection menu and the Class Tree. The first tank will always have a light blue background. The second tier will be green. The third tier will be red. The fourth tier will be yellow. The fifth will be dark blue. The sixth tier will be purple. (this order can be found in the tank’s add stats buttons which are color-reversed, or based on the game mode selection, which uses that color pattern, and is only used for the Factory as of now). Regardless of speculation, these colors do not seem to represent anything other than a simple order pattern to more easily distinguish tanks apart when quickly selecting an upgrade for now.
The following are Builds for certain types of Tanks. Working with the 33 skill points you can use or have already used at level 45, here are some skill point investments according to the type of tank, such as sniper, octo tank, etc. Numbers correspond to the order of the stats, top to bottom: Health Regeneration, Max Health, Body Damage, Bullet Speed, Bullet Penetration, Bullet Damage, Reload, and Movement Speed. Most builds are centered around a specific tank, or tanks shown in their title. When using that build, make sure to choose your tank upgrades correctly.