A common specification for microphones, max SPL indicates the highest sound pressure level a mic’s electronics can handle before the onset of distortion. Normally, this spec is referenced to 0.5% distortion at 1 kHz. Keep in mind that the presence of an attenuator switch on the mic may allow an increase in the volume level the mic can absorb before distorting.
Obviously, this is an important spec for many applications – if the mic is going to spend its life in front of a screaming Marshall stack, or in a kick drum, it must be able to adequately deal with the volumes it will be seeing…