When are you "in terrain"?

As the topic says, When is a model considered to be in terrain? I quick scanned and only came up with pg 102 of BRB that says a model must be within the borders of the forest to get a cover save. I have been told that being in base contact with the terrain border is merely enough to be considered "within" terrain.

What is the general consensus or is there some rule/FAQ I'm missing?

Thanks.

Thats the sort of stuff you need to agree before the game,
All the terrain I use is mounted on a piece of wood.
On the wood, in terrain, off it, out.

Did he take a difficult terrain roll?
No?
Then he wasnt in the terrain clearly...