asterisk users - Sep 2005 - show queue callcenter output?

Hi,

Can some one tell me what is the meaning of all the fields of show queue 
callcenter? for example in my system it gives:

callcenter   has 0 calls (max unlimited) in 'roundrobin' strategy (33s 
holdtime), C:429, A:12, SL:0.0% within 0s

How is the holdtime calculated? what is A and SL?

Also how can I see which of my zap interfaces are busy currently? I did 
a zap show channels I get this output, but no indication as to which is 
busy and which is free.

rapid*CLI> zap show channels
    Chan Extension  Context         Language   MusicOnHold
  pseudo            ivr
       1            ivr
       2            ivr
       3            ivr
       4            ivr

Thanks and regards,

raj