I'm searching what would be the cover of a creature that is on a platform of about 10ft. Is it like +2 or +5?? The PHB and DMG just talk about walls and corners but nothing from platform.
Use the same cover rules as presented in the PHB, just translate them into the vertical. It might help to draw out a side-view of the arrangement.
At a guess, I'd say it probably works out to no cover from a target more than 2 squares away, cover from a target two squares away, and superior cover for someone adjacent to the platform's edge.