Points > Grid Sample (applicable to unordered data only) reduces the number of points in the point cloud by eliminating all but one point at uniform intervals on all three axes. Imagine a three-dimensional grid that is superimposed over the object, and the point that is closest to the center of each cell is maintained.
The controls are:
Spacing - specifies the width of each cell of the imaginary three-dimensional grid.
Define Spacing by Selection checkbox - click two points on the model, and the distance between those points specifies the width of each cell of the imaginary three-dimensional grid.
First Point radio button - specifies that the next point you click in the Viewing Area is the first of two points.
Second Point radio button - specifies that the next point you click in the Viewing Area is the second of two points.
OK button - eliminates the points and closes the dialog.
Apply button - eliminates the points and leaves the dialog open.
Cancel button - closes the dialog without eliminating points.