A Part is a reusable piece smaller than a whole Pattern. Think of a lower-third, an animated logo sting, or a captioned quote block.
Why Parts
Some pieces turn up in many Patterns. Make the piece once, as a Part, and drop it into each Pattern. Fix it once and every Pattern gets the fix.
How they relate
A Part is made of Stitches. A Pattern is made of Stitches and Parts. Locked Stitches inside a Part stay locked wherever the Part is used.