Each unit has it's capture rate, the more it is, the faster it can capture the point from the enemy. Usually infantry is the best capper. There are 2 types of capture rate - offensive and defensive. If a point is under enemy's control, your units' offensive capping rate works. If enemy tries to cap from you then defensive capping rate works.
Heavy tanks have higher defensive rate, and having them defend a point can often prevent more numerous enemy forces from taking over the point.