What does nav_msgs/GridCells represent?

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"an array of cells in a 2D grid" isn't a particularly clear description.

1. What defines the spacing of the grid?
2. Is the array one or two dimensional? So many ros types use a 1D array to represent a 2D grid, so the type alone is not sufficient documentation
3. What does cells[i] represent?
4. Do all cells have to be XY-planar?
5. Do all cells have to be planar at all?
6. Do the cells even have to align to a grid?
7. is cell_width measured from the center or the corner?