I'm not familiar with this head unit first off. What you're describing is a 3 band parametric equalizer. Each "band" has 3 different variables to it. "Center frequency", is just that, the "middle" of the range of sound you are adjusting.
eg. A 100hz center is going to be effected the most by adjustments. Frequencies above and below 100hz will also be boosted or cut. The amount of effect around it is called "Q factor". A higher Q number, say 7 means the area effected around the center is "narrow" or "tight". In other words, if you cut output at 100hz by 6 decibles in this mode, you may only be down 1db at 70hz., and at 130hz. [just an example] A low Q of say .5 in the same situation may have you down 4db. at 70/130hz., and still may be affecting frequencies out to 25/280hz. from the 100hz you adjusted at. [again, I'm making these examples up here.]
Then, obviously you have some kind of adjustment to "boost" or "cut" the frequencies you select. These types of equalizers are the best in terms of true tuning ability, however they are also the most difficult to set properly because of all the variables. I can't imagine one with continuously variable frequencies/Q factors etc. in a head unit, but these days who knows. How many "center" and "quality" options are in each of your three "E+ settings"?
ps. Is all this clear as mud yet? Sorry if I went off in left field for awhile.
