Setting a tank to “primary” is a practical way of sorting the vehicles in your Garage on World of Tanks, so that the tanks that are your favorite (and played the most) are easily available on the left side of the vehicle carousel. That way you do not have to look for your most favorite tanks too long.
Keep in mind that the more vehicles you set to “primary”, the less effective this system becomes.
You can tell a vehicle is set to “primary” by looking at its icon in the carousel. Primary tanks have orange ribbon in the right bottom corner of their icons.
You can set vehicle to “primary” by right clicking on its icon in the carousel and choosing “Set as primary”. Once a vehicle is set to “primary” it can be set to “reserve” by repeating the same steps, removing it from the left “priority” side of the vehicle carousel.