The Snap To Grid icon snaps a supported auto point feature to the 3D grid display in the Graphic Display window. As long as the Auto Feature dialog box is opened to a supported auto point feature, you can use snap to grid even when the 3D grid is not showing.
To turn on the grid in the Graphic Display window, click the 3D Grid check box in the View Setup dialog box, or click the Show a 3D Grid in the Graphic Window icon on the Graphic View toolbar.
Supported auto point features include all auto point features except for Corner Point and High Point.
With this icon selected, if you then select a point on a CAD surface, the XYZ value snaps to the grid.
With the 3D grid turned on, the selected point snaps to the visible grid. For example, if you are looking at the part in a Z+ view, then the X and Y grid lines are used. In general, whichever axis points the most towards you, the other two grid axes are used for the snapping.
With the 3D grid turned off, the selected point snaps to the invisible grid lines based on the feature normal. For example, if you select a point for a surface point feature, and the surface normal at the selected point is (1,0,0), then the Y and Z grid lines are used for the snapping. In general, whichever axis value of the feature normal is largest, the other two grid axes are used for the snapping.