Snap to Point is a function to move objects or other position parameter to a specific location within the viewport.
It’s possible to snap to subpoints from:
Snap the white tracker object to a corner of the cube.
If this icon is visible in the inspector the Snap to Point function can be used.
Click once on the icon and choose the object from the menu which should be used to snap to.
By click and hold the left mouse button it’s possible to select now the subpoints from the seleted object within the viewport.