What they do is: create per-interpreter variables when used as per the example. The "d" fetches the context. The "p" one can be used to pass it as a parameter. The "a" one is used to received it as a parameter. Use the "_" variants if it's followed by another parameter or argument. This is all explained.
You said that you already knew that, you said you didn't want me telling you what you already know, and you're always saying you didn't mean what I read, so I tried guessing what you actually wanted to know.