The minimum input impedance of the external preamp the microphone is connected to. A condenser microphone consists of a capsule and a microphone body having an inherent preamplifier or buffer stage. To get it to work it has to be connected to an external microphone preamplier or microphone interface.
The microphone should be able to deliver a decent voltage into the input of this external preamplifier. However, if the load is too low, the microphone signal will be reduced (less SPL handling). The higher the input impedance, the higher the possible output voltage the CPX preamplifier can deliver. For best performance it is usefull to know about Phantom power, the resistor network used and the input impedance of the microphone preamplifer the mic is connected to.
iSEMcon defined and validated the different Phantom powering options for the CPX12-series microphone, as well as the lowest impedance possible. For product reliability CPX12 output stage/buffer is short circuit protected for all Phantom power options as mentioned in the microphone specs.