Visibility
Some rules (especially the rules for making Ranged Attacks) ask whether a model is Visible to another model. One model can see another if you can trace an imaginary line from any part of one model to any part of the other model.
A model’s Base is ignored when determining Visibility, as are weapons, equipment, and protruding pieces of clothing, such as helmet ornamentation. This line cannot be traced through other models or Terrain Features, except Decorative Terrain. However, a line does not have to be traced to every part of the model; as long as it can be traced to one part of the main body, the model is Visible.
Target Visibility
When models make Ranged Attacks, the rules refer to Targets that are either Wide Open or Obscured.
Wide Open
When checking Visibility, a model is Wide Open if a line can be traced to all parts of it (not counting its Base or any ignored parts mentioned above). This gives a bonus to the attack. Remember that Decorative Terrain Features are always entirely ignored, and so can be moved aside when checking if a model is Wide Open.
Obscured
A model is Obscured if any part of it is hidden by any part of another model from the same crew. This will make it harder to select the model as the Target of a Ranged Attack.