"Modified" EPS is the same basic material as "regular" EPS other then the fact that M-EPS is self-extinguishing. This material will only burn in the presence of an ignition source. When the ignition source is removed the EPS self extinguishes...
The requirements of all the major code organizations for "modified" (fire retardant or self extingushing) EPSis that they have a flame spread of not more than 75 and a smoke development rating of not more than 450. The standard test for this is ASTME84-87.
source: Fire Prevention Handbook - 17th edition.