The Expansion function is used to add a new fragment with new grid cells to the grid as an expansion of the selected fragment. You can determine the number of grid cells which shall be added in each direction.
For example, if you choose an expansion of one in plus X, the expansion fragment will contain one grid cell following each final grid cell of the original fragment in the plus X direction.
If the Overwrite Grid Cells of other Fragments check box is enabled, other fragments, or parts of other fragments within the section of the new fragment, are overwritten.