Amapi 3D allows you to snap the cursor onto an existing point or a specific point on a segment (relative to the nearest end).
Check the designated box in the “Preferences – Constraints” dialog box to activate/deactivate this feature of the cursor.
q Onto a point
q Onto a segment
1. If you checked the “Snap onto a point” box:
The shift key will have the cursor snap to existing points.
2. If you checked the “Snap onto a segment” box:
You will need to specify the length of the spacing onto which the cursor will snap.
The shift key not only snaps the cursor onto existing points if you specified it in the Preferences - Constraints, but will also snap the cursor onto user-defined spacing along the segments (spacing relative to the nearest end).