Equipment improves the effectiveness of the tank in combat. Unlike consumables, equipment is a one-time cost addition, with each having its own advantage; however, they come at a high cost. Equipment availability and costs vary by tank tier, class and nation.
Equipment can only be used on the tank it is installed on. If you want to use it on another tank you need to sell it first when its removable equipment. This option is available when clicking the installed equipment in the garage screen. Demounting removes the item from your tank and sends it to your Depot, making it available for use on another vehicle. Most equipment though require a small amount of gold to demount, as is explained below.
Purchasing from your vehicle's equipment panel is the best way to ensure compatibility. Only use the store to purchase equipment for vehicles you don't own yet and only if you have a firm understanding of the equipment system.
With respect to demounting equipment, two types of equipment need to be distinguished:
Removable Equipment that can be demounted for its full money. Complex Equipment that is permanently installed on your tank. This is indicated in game by a gold nut on the equipment icon. Complex equipment can be removed from a tank or disposed of in one of two ways: Pay 10 gold to demount the equipment and return it to the depot (from where it can later be mounted on another tank) Destroy the equipment, which frees the equipment slot, but no compensation is given for the lost equipment.