Lets say you are out detecting and keep digging up pennies...as if someone threw them everywhere and you need to ignore them to get through the area...you would be able to modify your pattern on the fly so that you could ignore them just for that section of beach.
You can also use it when creating patterns on your machine.
I have yet to use the button while detecting. Personally if I want to ignore something I want to hear it and then choose to ignore it or not so my current pattern is to accept everything.
Funny you say that, when at myrtle I was talking with the front desk person. They told me there was a couple that would come down twice a year. They would save pennys then go out at night and throw them all over the beach in front of the hotel. The next morning they would come down for breakfast and fill a jug with coffee. Return back to their room and sit on the balcony drinking coffee watching the MD'er go crazy detecting pennys!
On most machines, that button can be used in a couple of ways. You can program it to just accept a number or group of numbers, or to reject that number so it won't register on your display or give you a tone. Say someone lost an earring and you want to find it. You have the other one for a sample. You scan it and accept the reading. Then you can ignore everything else and just look for a target that matches that reading. The reject setting it helpful when you have an area with a ton of the same junk target, or, you're getting false readings in a given area that is driving you crazy. You reject that number and your machine will ignore all targets with that reading.