Low impedence = around 8 - 17 ohms
High impedence = around 300 ohms
Civil aircraft VHF radios use high impedence, as do UHF CB type radios.
Military aircraft use low impedence setups.
Low impedence mics, M87 and M101, can be amplified to high impedence with a plug in amp. Both are dynamic mics.
Electret mics tend to be high impedence. Stand to be corrected.
Seems to be important in multi crew to have every one the same. Seems electricity, like water, takes the easiest way, so whoever has the lowest impedence gets all the electricity.
Thats how it was explained to me (I think)
Best bet? Have a look at what everyone else is using and get the same.