Line of Sight Rules

You can trace Line of Sight from any part of your model to any part of the target unit. For the purpose of targeting I recommend using 7ths targeting rules (I.E. wings, antennae, banners) do not count as a part of the model, meaning you cannot draw los from or too these bits.

Targeting Occupied Terrain Occupied Terrain is any terrain that has a unit within the terrain feature. Units that occupy a Terrain feature can see and be seen through it. Units that Occupy Terrain gain Cover from the terrain. A unit is considered to be occupying the terrain if all of it's models bases are at least partially within the terrain or meet it's other requirements. Models that do not have a base must be at least 50% within the terrain to be considered to Occupy it.

Intervening Terrain Intervening terrain is any terrain that sits between you and the target unit but is not occupied by the target unit. You can trace LoS over a single piece of Light terrain. A second piece of Light terrain and/or dense terrain will block LoS normally. Targeting a unit over intervening Terrain confers a -1 to hit penalty.

High Ground If your unit is on a piece of raised terrain they may have high ground. A unit with high ground can ignore all terrain and los blocking terrain features when targetting units on a lower level so long as they can still actually trace line of sight to the unit. To repeat, you still need to be able to trace line of sight, but the target unit would gain no benefit from any intervening terrain.

Intervening Units If you cannot trace LoS to your target unit without tracing a line through an enemy unit the intervening unit counts as Light Terrain. That means if your target unit is behind both an enemy unit and a piece of Light terrain that unit is untargettable because your LoS is blocked (just like 2 pieces of light terrain).

Monsters, Vehicles, and Titanic When targeting any unit with the MONSTER or VEHICLE Keyword you ignore any intervening units when tracing Line of Sight treating them as Open Ground. When targeting any unit with the TITANIC keyword you ignore all intervening units and Light Terrain treating them as Open Ground. In addition treat all Dense Terrain as Light Terrain for the purpose of tracing LoS on TITANIC units.

Flier Units with the Flier battlefield role can be targetting freely treating all terrain as Open Ground so long as you can still trace Line of Sight.

Terrain

All terrain has 3 features.

1) Line of Sight
2) Cover
3) Difficulty

1] Line of Sight

There are 3 degrees of effect terrain has on LoS.

-Open Ground: No effect on LoS. This terrain piece can be shot over as though it was not there. Example: A water pool or river.

-Light: Blocks LoS to some extent. You can draw Line of Sight over a single piece of light terrain. A unit cannot draw LoS over 2 pieces of light terrain.

-Dense: Dense Terrain blocks LoS entirely.

Cover

All terrain has a cover value that is a bonus to your Sv roll (Ex. +1). This bonus is granted to any unit entirely within or meets the requirements of the terrain feature.

Difficulty

All terrain has a difficulty value. This value is a penalty to the Movement Value of any unit that enters or attempts to move through the terrain. It is possible the Difficulty of the terrain is a 0 meaning it does not impact movement at all. They may also have special considerations such as "Impassible to VEHICLES".