A SIG is a Special Interest Group - which would be pretty well anyone's guess.
For HRI however - a SIG has a special meaning in that:
SIG's are the "knowledge gods" for their special interest - and you don't argue with knowledge gods - even if you know they're wrong. Join the SIG of your choice and you too can become a "knowledge god" and have everything you say be right because you are SigMaster!!!
(sort of like that unix "i am root!" t-shirt - with upper case!!)
One of the primary products of SIG's in HRI are the SIG sites - which hold the collective knowledge of the members of the SIG - at least for projects they have done recently. Here the SIG members dispense their wisdom to the ignorant masses (i.e. - non-members of the SIG) and correct our misunderstanding about "what is Voltage?" or "what's a Balance Sheet?" or "what is TCP/IP?" <hint - it's not IP - see sig:leg for that>.
HRI SIG webs are (so far) all in project hri07e's km6 format - i.e. they all have the same structure - so if you are looking for "key codes" - they are all located in the same section of the km6 sig site - no matter what sig you are reading. There is no room for "artistic freedom" or "thinking outside the box" in the km6 sites. If you want to do that - invent your own version of a sig site and see if you get any takers.
-check here for a list of SIG's