In Practice
Since each microphone manufacturer can define both the extent of the deviation from the ideal frequency response and the ideal frequency response itself, the specification of the frequency range as a comparison value between different microphones and microphone manufacturers is hardly to be used. The much better and more objective property for specifying the transmission characteristics of a microphone is the frequency response, from which ultimately the frequency range is derived.
The frequency response of some microphones, especially for the low sound spectrum, depends strongly on the distance between the sound source and the microphone (see "proximity effect" of pressure gradient microphones). As a result, the transmission range can not be unambiguously specified as the lower limit at a fixed dB value. This limit would change dramatically if the distance changes. Some microphone capsules, such as the MK 4VXP, are designed specifically for short distances to the sound source and feature intentional bass reduction. A fixed dB limit in the frequency response, which is usually measured at a distance of 1 m, would not be effective here. This would thus obtain a lower limit frequency which lies above the part of the sound spectrum in which the microphone capsule is actually still used. Therefore, here a limit is assumed, which would occur at a much closer distance. This could imply e.g. a -3 dB threshold frequency when the proximity effect at this close distance increases the low frequencies of the frequency response curve. Since the exact dB value is not specified here as well, the value must be questioned even if the distances to be measured are specified (as it is in the current IEC 60268-4 standard).
For the upper realm of the sound spectrum in a microphone recording, the manufacturer can specify at which deviation in dB relative to the level at 1 kHz, the upper limit of the frequency range is defined. Here, the distance usually has a negligible influence on the frequency response, so that this limit can indeed tell something about the transmission characteristics of the microphone.
Since the value of -3 dB (relative to the frequency response value at 1 kHz) is a common threshold for the frequency range of loudspeakers or amplifiers, for the SCHOEPS technical specs also -3 dB was chosen for the upper limit of the frequency range. That frequency, at which the frequency response intersects this -3 dB limit, is the upper limit of the frequency range.
