Focusrite ISA One Analog

[river]

They offer different ratios of impedance between the mic and the pre, which can produce some subtle to very noticeable changes in the frequency response of the mic.

[elektrovolt]

the different input impedances give a different load on your microphone.
with higher impedance there will be more gain and high end. however, this effect can be more or less (sometimes even not) audible.

I've got an Isa one and the impedance selector does nothing with my old C414EB, but is really audible with my ribbon mic.

choose the setting that sounds best for your taste and purpose.

[SeezeTheWorld]

I have one and typically just set it at the ISA 110 setting. I have a Blue Baby Bottle mic running thru it.

Never used the other settings but now that others have posted I'm going to try them out and see if I notice a difference.