They did an FAQ on this.
Page 248 – Ruins
Change the first paragraph of rules text to read: ‘Unless they can Fly, Vehicles, Monsters, Cavalry and Bikers can only be set up or end their move on the ground floor of ruins.’
Change the third paragraph of rules text to read:‘Infantry units that are entirely on or within a ruin receive the benefit of cover. Other units that are entirely on or within a ruin only receive the benefit of cover if at least 50% of every model is obscured from the point of view of the shooting model.’https://www.games-workshop.com/resource ... ok_ENG.pdf
There's two requirements. For ease of discussion, we will use a devilfish tank as our example.
1) this devilfish must be on or within the ruin. So it is either on top of it, or touching it. The definition of "within" is to be touching the terrain in some way. So if an engine of the devilfish is physically touching the ruins, you're within. It is explained in this FAQ post.
Q: Can you clarify what the difference is between ‘wholly within’ and ‘within’ for rules purposes?
A: If a rule says it affects units/models that are ‘wholly within’ then it only applies if the entire unit/model is
within. If it just says ‘within’, however, then it applies so long as any part of the unit/model is within.
For example, units gain the benefit of cover if every model in the unit is either on or within terrain. So long as all the models in that unit are either on or partially within the terrain, they gain the benefit of cover.
2) You have to be 50% obscured from the point of view of the shooting model. That does not mean that the terrain the devilfish is on has to be the one that is obscuring the shot. Anything that obscures 50% counts.
Q: Do units that are not Infantry (Vehicles, Monsters,etc.) gain the benefit of cover from woods, ruins etc. if they are at least 50% obscured by that piece of terrain but are not actually on or within it?
A: No. Unless they are Infantry, such a unit must meet the two following conditions to gain the benefit of cover:
• All of its models must be either on or within the terrain.
• The unit must be at least 50% obscured from the point of view of the firer (note that it doesn’t matter what is obscuring the target, only that it is obscured).