ISDE legal means 93 decibels or less per the rules they use to measure. It is spelled out in the AMA rulebook for qualifiers I believe. If a qualifier is run on USFS land then it will need to be an SA also, not just meet the 93 db requirement.
Spark Arrestor is a US thing. There are several ways to determine if you have one. One, look for the USFS writing on the side. It is on the inside towards the tire on the FMF and PC silencers I have seen. Two, look for the Krisman (sp?) marking and turbine-style innerds. Three, look for a screen that the exhaust passes thru, a wire mesh-type screen. And a final definition, per the AMA book, a rod cannot pass more than 18" into the exhaust.
This last one seems to be the hokiest IMO. For example a shorty silencer with a bend right in front of it won't let the 18" rod go all the way in when you have a small diameter stinger on a 125 and 200. Same for a downturned tip installed in like a stock YZF can - no SA but the rod won't go 18" cuz of the sharp bend. One could theorize that this bend in effect arrests the sparks, but I would prefer not to get a ticket from old Smokey and like to have the writing on the side.
Hope this helps.