Editing Warhammer 40,000/9th Edition Tactics (section)

=====Cover=====
Light cover improves the armour save of the unit that is in cover by 1 against Shooting attacks regardless of which phase that Shooting attack is made in or whether the target or firing unit is in combat, and Heavy cover may give the same armour save bonus against close combat attacks. A model cannot claim multiple cover saves, it is either in cover and receives the bonus, or it is not, and it does not benefit. If you have a unit that is partially eligible for cover the whole unit will not benefit, but if you remove all the models from the unit that are not eligible the unit benefits immediately; therefore, it can be beneficial to roll your saves one at a time and pick off the ones outside cover first. 

*Certain units and terrain types (see terrain 101 above) have special rules which can affect chances to hit or visibility.