The fact that they are gray means they are 'Inactive'. They must be activated if you want to apply relationships to them, include edges from them in sketches, etc.
The top one (larger box) is the symbol when a part is fully constrained, through assembly relationships you have placed. The bottom one (I'll call it box with parentheses) is the symbol when one or more of a part's six degrees of freedom is not constrained, and the part can move.
The brackets are actually to show movement. You have the original position corner, the current position with the cube and then the destination corner the second bracket. I just noticed that is the icon form move component in NX.
Don't think this is an original idea...this is basically trace lines showing movement and goes all the way back to pre-historic art!