My mixer has four EQ knobs per channel. Two of them are standard EQ at 10khz and 100hz. The other two are for the mid range, but they're different. The lower knob sets the cut/boost, but the upper knob sets the frequency from 250hz to 5khz. The manual says this knob "sets the variable mid frequency" while the gain knob "sets the amount of attenuation or boost for the range", except the frequency knob isn't a range, it's a discrete number. Hence the confusion. Also, the manual never uses the word "variable" outside of that one sentence.
I ASSUME what that means is that the cut/boost knob creates a normal or near-normal curve centered on where the frequency knob is, but the manual doesn't make it clear and to be honest I can't really hear an obvious, clearly correlated difference when I sweep the frequency knob around.