Hi Riley,
Thanks for your answer.
Obviously, I have already used the manual prior to posting here.
The manual is very short about this option. It only references "/etc/reportbug.conf" config file. And this file references 'reportbug --bts help' like I said in my first message.
But I don't know yet how to send a bug report to another BTS server different than servers recognized for 'reportbug --bts help' (guug, debian, ubuntu).