The variant system allows you to create, well, variants of your tanks/planes/ships with land/air/naval experience. You can improve them in 4 Categories by default, which vary depending on the type of equipment you are looking at. Carriers for example have armor, reliability, deckspace and engine, while submarines have stealth, engine and torpedoes as their available categories. Tanks have armor, reliability, engine and armament. It should also be noted that those are modable and that they are capped at 5 upgrades per category. That means you can only apply 5 points in armor, deckspace, reliability and engine for an carrier each, for example.
Here you go.