Hi,
SNR is the single number rating used in the EU for hearing protector guidance, there is this overall rating figure and further figures for High frequency (H), Mid frequency (M) and Low frequency (L) protection. It allows you to estimate noise attenuation by the protectors, its value is determined following an ISO standard (see L108 Control of Noise at Work guidance). I think NNR is the USA equivalent i.e. the noise reduction rating which is an estimate of noise reduction by protectors but the NRR value is determined in a different way.
Steve