Infos about key system (shared line appearance) with SIP

Hi all,

I’m looking for documentation, how key system (shared line appearance, multi line appearance, …) with SIP works in Asterisk.
All I found is … +%28SLA%29. Is there a more detailed documentation available how call features (e.g. consultation call, hold, retrieve, …) are handled with key systems in Asterisk?
It would also be very helpful for me, if s.o. knows, where I can get traces and call flows about this topic?

Thanks in advance