The problem is there is no single NEC rule applies to your question; however, the general rule is that equipment must be labeled for each Class, Division, Zone, Group, etc. See Section 500.8(A) and (B). There are some ?Special Allowances? listed in 500.8(B) (6). There are also provisions for Division 1 marked equipment in Division 2 and ?Division? rated equipment in ?Zones.? But there are no provisions for equipment marked for one ?Class? to be automatically used in another.
The IS marking should however make the device permissible in Class III assuming it has a "T" rating less than that specified in 503.5
Thanks for the reply, I sent a e-mail to the manufaturer just to cover myself.
And their reply was, that no it is not rated for class 3. Even tho it does fall
within the safe areas. So in a strict enforcement area, if it's not on the tag, it doesn't get used.