That One Guy said:
After busting out my PHB, I think SS's right. I'd say concealment stacks up such that concealment + concealment = Heavily Obscured. Heavily + concealment = Totally Obscured, thus -5 at melee and range, thus effectively invisible.
Soo written out...
Lightly Obscured/Concealment = -2 Range, -0 Melee (An adjacent square does not get obscured by Shadow walk, fog, low-light, etc.)
Heavily Obscured/Total Concealment = -5 Range, -2 Melee
Totally Obscured/Total Concealment or effectively invisible = -5 Range, -5 Melee
I don't think you can get more concealed than invisible.
Other than being on the other side of a wall.
Bwah? Where are you getting those statements from? Obscurement is an outside source that blocks a PC's or NPC's view, such as fog, rain, ect. The only spell I can think of that makes obscurement is Sudden Storm. Obscurement causes Concealment, Concealment does not cause Obscurement.
Concealment = Hard to see: -2 Range and -2 Melee.
Total Concealment = Unable to be seen: -5 Range and -5 Melee.
For Obscurement:
Target is in a lightly obscured square and you are attacking with melee or ranged:
Attacking the target in a non-adjacent square = Target has Concealment
Attacking the target in an adjacent square = Target has Concealment
Target is in a heavily obscured square and you are attacking with melee or ranged:
Attacking the target in a non-adjacent square = Target has Total Concealment
Attacking the target in an adjacent square = Target has Concealment
Target is in a totally obscured square and you are attacking with melee or ranged:
Attacking the target in a non-adjacent square = Target has Total Concealment
Attacking the target in an adjacent square = Target has Total Concealment
If the DM wants to, he add obscurement on the battlefield together for a higher level of obscurement. Say a square has both rain and dim light. Dim Light grants the square lightly obscured status, and rain grants the square lightly obscured status also. Just bump that square up to Heavily Obscured and take it from there. If the square is in Dim Light and Heavy Fog, the DM can bump it up to Total Obscurement if he or she wishes.
For Concealment + Concealment, I would say to take the best concealment and ignore the rest. So a Warlock activates his Shadow Walk, but he is also in a Totally Obscured square, has Total Concealment. A Warlock activates his Shadow Walk, but is also in a Lightly Obscured square, has Concealment because the Concealment from Shadow Walk and the Concealment from being in a Lightly Obscured square would not render the target Totally Concealed.
For Concealment + Cover, I would say to add their penalties together. Cover is a part of Line of Effect, while Concealment deals with Line of Sight. Just remember that being partially/mostly behind a low wall or a corner for Cover, does not grant Concealment. Partially meaning they can still attack a target back without moving.