In addition to the targets, there are several other snap options that you can set. They affect the behavior of snapping in different ways, and can be set independently of each other.
All of these options are available on the Snap menu, as well as on the Options tab of the Snapping preferences that opens
when you choose Snap 
Snap Options.
The snapping region is a box around the mouse pointer that defines the targets to which elements can snap. The element you are moving or adding will snap to a target only when the target is within this box.
If you deactivate the region, elements snap to the closest target no matter how far away it is on screen. Note that deactivating the region can slow down snapping on large scenes because all visible objects must be tested for snapping.
The Region Size is the size of the snapping box around the mouse pointer. The size is defined in pixels, so it is larger in Softimage units when you are zoomed out than when you are zoomed in.
To increase/decrease the region size by increments
Press Shift while scrolling the mouse wheel. Each increment is 10% of the current radius. This works only if you are not using the mouse wheel to zoom — see Mouse Wheel in Camera Preferences [Preference Reference].
Snap to Unselected Objects Only
The Snap to Unselected Objects Only option prevents components from snapping to other components on the same object.
You can activate this option, for example, if you want to snap an object's center to a point on another object while preventing the center from accidentally snapping one of the object's own points.
Snap to Tagged Components Only
The Snap to Tagged Components Only option restricts the targets to the last set of components of a given type that were tagged (selected) on an object. You can tag components such as points on the target object beforehand, and then when you move an element such as component on another object, it will snap only to those points.
Set the snapping targets and other options as desired.
For example, if you want to snap to selected points on a polygon mesh, make sure that the Points target type is on, that the Points target subtype is on, and that the filter is set to All Objects or Polygon Meshes.
Activate snapping, and then use any tool that supports snapping. The element you are manipulating snaps only to the components you selected on the target object, and ignores unselected components. The tagged points on the unselected object are highlighted for reference.
Use 2D Snapping in Orthographic Views
The Use 2D Snapping in Orthographic Views option snaps an element only in its own plane with respect to an orthographic camera when manipulating in View mode.
An element's Y position is not affected when snapping in the Top view.
Its Z position is not affected when snapping in the Front view.
Its X position is not affected when snapping in the Right view, and so on.
When this option is off, elements snap in depth in the orthographic views, except when snapping to the grid.
Snap to Closest Segments/Midpoints Only
The Snap to Closest Segments/Midpoints Only option prevents elements from snapping to elements that are facing away from the camera as determined by the direction of their normals. This option applies only when snapping to midpoints, edges, or knot curves on closed shapes.
Snap to Visible Grid in Camera Views
The Snap to Visible Grid in Camera Views option snaps to the floor grid instead of the view plane grid when snapping to the grid in perspective views.
When this option is off, the snapping plane passes through the camera interest and is parallel to the viewing plane — as you manipulate elements, an overlaid orange grid shows the target grid.
