Its true - it resembles a little master/slave in specific scenarios: ECU do not broadcasts its P/N during normal work, so you need to explicitly ask for it.
Its sometimes hard to say which entity is master and which is slave though.
Please do not stick with my nomenclature. It may be just wrong naming.
If you want to understand this better - there is excellent introductory page here:
https://www.csselectronics.com/pages/obd2-explained-simple-intro
In general its just one particular entity (one ECU) is little shisofrenic
it is listening on one specific ID (I called it send ID) but it produces messages using another one - I called it "response ID".
There is another thing: one ECU (lets use ICE motor ECU as an example) do not listen for any message to set specific motor RPMs. It listens only on messages from "his known colleauge" which can be throttle pedal.
