On the left there is a Generating Station with a line going into the “Generator Step Up Transformer”. This is the Generation portion of the diagram.
From there it goes to the Transmission Customer (138kV or 230kV) and to Transmission Lines (765, 500, 354, 230, and 138kV). This is the transmission portion of the diagram.
From the Transmission Customer and Transmission lines it goes into a Substation Step Down Transformer for distribution. This is the Distribution portion of the diagram.
From there it goes to the Sub transmission Customer (26kV and 69kV), the Primary Customer (13kV and 4kV), and the Secondary Customer (120V and 240 V).