basically it is setting your unit up to read 0db which will output 75db read on the meter. you can adjust up or down from that "reference" mark. what does it really matter if you are setup for reference standards? nothing to me, id prefer to have it the way i like it, but it is a good starting point to find out what you like...

there are different ways that volumes are displayed/controlled, so you may not be able to adjust your unit to be 75db at 0db on the volume display. also louder could be -db or it could be +db. sometimes there are options to change the volume display. its like absolute or the normal way i think.

more than anything, id use the test tones to make sure each channel is delivering the same amount of sound potential to your listening position. what level that you set them to is irrelevant, but you do want all of the channels to be equally loud at the listening position(s).