After an element has been designated as a part, that part can be divided into smaller parts, either by sketching division lines or by selecting reference elements that intersect the part.
Video: Create and Divide Parts
Part panel
(Divide Parts).
(Edit Sketch).
The Draw panel displays with the Line tool selected.
(Finish Sketch Mode) to exit the edit sketch mode
(Finish Edit Mode) to exit the edit sketch mode.
Part panel
(Divide Parts).
References panel
(Intersecting References).
(Finish Edit Mode) to exit the edit mode.