[tor-onions] Criteria for intro point, rendezvous point selection?

[Paul Syverson]

We were trying to figure out some criteria for selecting intro points
and rendezous points as well as circuits to connect to them and had
some questions we couldn't determine from our poking around for answers.
Does anyone know for current stable code (and, e.g., post 224 etc. if
any of this is expected to change) the following?

Are guards ever selected for intro points?
Are exits ever selected for intro points?
Are guards ever selected for rendezous points?
Are exits ever selected for rendezvous points?

Are client circuits to intro points selected from prebuilt circuits?
If so, is the third hop an exit relay? Or is a prebuilt circuit
cannibalized in some way to avoid using an exit?

Are client circuits to rendezvous points selected from prebuilt circuits?
If so, is the third hop an exit relay? Or is a prebuilt circuit
cannibalized in some way to avoid using an exit?

Fuller answers (e.g., 'Yes. guards can be selected as intro points, but only
in accord with their chance of being selected as a middle relay.') and/or
where to find these in documentation appreciated.

If someone knowledgeable feels this is better directed at tor-dev, please
let me know (or simply reply there rather than here).

aloha,
Paul