Being in cover can add one or two black dice to defence, although I operate with three levels - light, medium and heavy cover.
Being concealed adds up to three black dice ro ranged attacks.
If you're hiding in shoulder high grass you would add 2 dice to any ranged attacks targeting you.
What if' you're peeking over a stone wall? Then you're partially concealed as being in shoulder high grass and thus should add +2 black dice to any ranged attacks targeting you because of concealment. On top of that you should also add at least 2 dice for cover, so total of four dice added.
I this wasn't true, it would be as good to hide behind a courtain as it would be to hide behind a thick stone wall.
I say that concealment and cover are cumulative
For being in a bunker and shooting out of a narrow slit I'd add +6 black dice to any ranged attacks targeting you.