Time Domain and Frequency Domain Amplitudes
Almost certainly the amplitude in the frequency domain is in RMS volts.

Divide the pk-pk voltage by 2 * sqrt(2) to get the RMS amplitude for a sine wave.

994 / (2* sqrt(2)) = 351mV.

You have to be careful with the way that various instruments will display amplitudes, they could be peak to peak, peak, RMS or in in decibels (dBV).