Customize menu > Grid and Snap Settings
Right-click any snap button
This command displays the Grid and Snap Settings dialog. This modeless dialog establishes settings and options for snaps, the home grid, and user-defined grids.
Controls on the Grid And Snap Settings dialog determine which snap settings are used when you activate snaps by clicking 3D Snap Toggle. Adjusting any of these snap settings does not automatically turn on snaps.
You can use objects within an XRef scene as snap references.