Microphone sensitivity is calculated as the output level the mic will produce in a known sound field.
RØDE and most other professional audio manufactures use a sound field level (or pressure) of 1 Pascal (or pa) = 1V, this equates to 94dB SPL. To test the microphone sensitivity we place it in a sound field of 94dB SPL and this is our reference level.
The microphone output is measured in millivolts and stated in dB compared to this reference level. As the reference level of 1 volt is normally above the output level of the mic (mV) the resulting number in dB will be negative. So, a microphone with a sensitivity of -40 dB is more sensitive than a mic with a rating of -60 dB.