Purge msg misunderstanding

We installed Messaging Server 6 (2005Q4) and we don't understand 
exactly how work the purge (/opt/SUNWmsgsr/lib/purge -num=5).
 
We keep the default configuration like following:
local.sched.enable = 1
local.schedule.expire = "0 23 * * * /opt/SUNWmsgsr/sbin/imexpire"
local.schedule.msprobe = "5,15,25,35,45,55 * * * 
* /opt/SUNWmsgsr/lib/msprobe"
local.schedule.purge = "0 0,4,8,12,16,20 * * 
* /opt/SUNWmsgsr/lib/purge -num=5"
local.schedule.return_job = "30 0 * * * /opt/SUNWmsgsr/lib/return_job"

We configured some expire rules (in the 
file /opt/SUNWmsgsr/config/store.expirerule) and these expire rules 
work perfectly each day at 23:00.
 
After the messagers are expired, we wonder which command will purge 
(remove permanently) these messagers from the disk (1.msg, 2.msg, ...)?
 
What is exactly the purge functionality (/opt/SUNWmsgsr/lib/purge -
num=5)? We know that -num=5 means that expired after 5 versions but 
what does it mean exactly?
 
When we lunch the purge binary without any parameter 
(/opt/SUNWmsgsr/lib/purge) no message is purged. Why?
 
Does this purge command log anywhere in a log file?
 
Thanks for your help!