A Part is The Central Idea or Concept of a Building.

HYPERMAP — Design Framework for Development of TUD Campus

The Parti is the diagramming of the essence and the core idea of a design agenda. The diagram can vary from a single diagram to a series of diagrams. Core ideas and agendas can be circulation, massing, programming, site relationship, public/private relationship, solid/transparent and many more. These diagrams help your panel and your audience understand your core ideas at a glance. The Parti diagram is a powerful tool in distilling the very essence of your ideas into one clearly understandable diagram.

Eisenman Iteration Diagram for House X
Eisenman & Derrida La Villette Garden Exploded Axonometric Diagram
Eisenman Diagram Series for Check Point Charlie Competition Berlin