Can exit relays be selected as middle or guard nodes?

Question

I was under the impression that exit relays are only ever used as exit relays, however, recently I read a paper that stated that exit relays can be selected as guards or middle nodes.

Is this true?
I run an exit relay and on Tor Atlas my relay is listed as having a probability of being selected as a guard as 0%. The same seems to be true for being selected as a middle.

Answer 1

It is true in the current case, but the bandwidth authorities make a scarcity calculation which depending on result decides whether exit relays can be used as guards/middle relays. If exit bandwidth grows they might get used as guards again.