cylinder block boring

If a single cylinder is found to be out of tolerance, can just that cylinder be rebored and fitted with a new piston / rings or must all be done?
if all must be done, is this for balance purposes?

You could, if you absolutely had to, bore just the one cylinder but it is generally considered a bad practice. The reason is that displacement = horsepower. That one cylinder will make slightly more power, need a slightly bigger jet, and make general tuning more difficult. I don't know this from experience because I've never bored just one. If one is out of clearance I always bore them all.

If you are talking stock pistons, it would not be a problem. They make .020" over and that wouldn't be noticeable.

Jay