The rectangle tool's behavior near the edge often leaves a pixel or two uncovered as the user's finger exits the touchscreen area (and touchesEnded registers a pixel or two from the edge).
This optional parameter allows the rectangle tool to snap to the edge with a configurable threshold (in px) - in practice I've found 15px is a generous value suitable for use by children.
The rectangle tool's behavior near the edge often leaves a pixel or two uncovered as the user's finger exits the touchscreen area (and touchesEnded registers a pixel or two from the edge).
This optional parameter allows the rectangle tool to snap to the edge with a configurable threshold (in px) - in practice I've found 15px is a generous value suitable for use by children.