Grids is what i call graphic intended for walls that allow to see trough holes in them and stop or allow bullets, projectiles, players, monsters, with rules that are different from those of a solid wall. Some examples:

A fence might stop actors but allow bullets and projectiles

A tent or carwash brushes might obstruct the view but allow everything to pass trough, 

A futuristic forcefield might be designed to allow players and monster to pass trough, stop energy base projectiles.
And maybe doesn't stop bullets because the author of a project decides aliens didn't conceive solid bullets when they designed said forcefield.

A couple of those cels are indeed solid, they represent a windows viewed from outside  when it is not necessary to design the corresponding room behind it, i kept them in this array for my convenience.