On certain agenda or runner cards you can place host counters. Two questions -

1. Are these one time use counters when played?

2. When can the corp play them and does it require a click to activate?

Thanks for any answers.

Yes, the counters do not refill and the cost of using the ability is to remove a counter from the agenda (each card will specify the cost of its own ability). This work the same for non-agenda cards with counters on them (like Imp or Raven). The card itself will tell you when to put markers on it and what to do with them.

The corp can use them any time paid abilities can be used (see timing diagrams in the rules and FAQ) without using a click (unless the cost of the ability contains the click symbol - no such card exists yet).

Actually….

http://www.cardgamedb.com/forums/uploads/an/ffg_executive-retreat-trace-amount.png

Actually…

Ok, I was behind the times about the existence of agendas with click abilities. I might have thought about abilities with both a click cost and a hosted counter cost (of which I am pretty sure none exist - but correct me if necessary). Nevertheless, I stand by the rest of my post about how and when to use hosted counters.