01
The menu: Source Capabilities
The source offers PDOs such as 5V up to 3A or 9V up to 3A. An APDO can describe an adjustable option such as PPS. A capability rating is not a measurement of present consumption.
02
The order: Request
The sink sends an RDO selecting an option and its requested conditions. For a fixed PDO, the selected position identifies the voltage. Adjustable requests can also carry a voltage value. Not every RDO is simply a recorded voltage/current pair.
03
Received, accepted and ready are different
GoodCRC acknowledges receipt. Accept accepts a proposal; PS_RDY reports readiness. Only the latter sequence can establish the corresponding supply conditions. The restaurant is an analogy: real PD has explicit states, timing and error handling.