Generally that number is supposed to be a temperature rating in Celsius degree. I usually only compare tires within the same brand though, because every manufacture does it differently. Even tires within the same brand are somewhat weird. the Sorex 36R has a HUGE temperature range it works in, but the 32R has a very small one.
RP 30s and Take of 27s have a pretty similar temperature range to each other, but its much different than Sorex's. Sweep generally has two major compounds in the EXP lineup. I use EXP30s when the track temp is under 90 deg F. Anything above that I tend to switch to EXP36s.
Just depends on the tire. But as others have said, the higher that number is, the harder the tire is.
Pro Spec America/Apex RC - Team ORCA - Awesomatix USA - Roche RC USA - AVID - Sweep - Protoform - Pro-One RC - Slapmaster Tools - Hebiki Design Works - Turtlemaster Racing - Seattle RC Racers - Timezone Hobbies