Dig In is a recurring ability used by a wide range of creatures, such as animals, vehicles and human combatants, although the effect in each case varies. When using it, the user trades mobility for a hefty defense bonus, becoming immobile but often entirely impervious to small arms. Note that while not all units are able to independently dig themselves in, and thus form an impromptu defensive position on-site during an operation, the vast majority of units can be fortified in defensive facilities constructed for them by other procedures.