If the majority of the models in the target unit are within Line of Sight, but behind models from another unit (friend or enemy) or terrain with half or more as high Line of Sight value as them, then an additional To Hit modifier is applied. For example, a unit where the majority of the models have a Line of Sight value of 1 in front of a unit where the majority of the models have a Line of Sight value of 2 will provide cover. However, the same unit will not provide cover to a unit whose majority of models have a Line of Sight value of 3 or higher. This works the other way around as well, so a model with a Line of Sight value of 3 will ignore the cover otherwise gained by a model with a Line of Sight value of 2 behind a model with a Line of Sight value of 1.
Cover will be classed as either soft or hard cover – see below. It is common for some models in the firing unit to have something in the way, and for others to have a clear shot. Where this happens, simply resolve the two sets of shots (and thus the two sets of To Hit modifiers) separately.