cross over

i think im missing something here. so a cross over cuts off and splits certain frequency ranges to different devices. i see cross overs that all have 20-400hz on there settings for the sub and highs. does this mean that the sub is a top cap and the highs is a bottom cap? so setting 150Hz on the highs will give <150hz and setting 150 on the sub section will give >150 hz to the sub? or does this limit the highs to 400hz? none of these cross overs go high enough do they? tweeters need a cross over at over 1KH dont they? how can a cross over only go up to 400?

good crossovers will have a switch on them that multiplies the crossover frequency range of the front output, this switch allows you to make a filter just to allow high frequencies that are meant for the tweeters to play.