Answer:
When you submit a request to Dial Before You Dig, they send out your requested details to each underground utilities provider that has subscribed to Dial Before You Dig and that has underground utilities in that area.
Those companies are the ones that then send out the plans to you, of their underground utilities, giving you a guide to what is there.
Please keep in mind, that although the Dial Before You Dig service is a brilliant system, there are still underground services that is not listed on the plan you get back. Also the plans are schematic drawings and should only be used as a guide.
For a more detailed explanation take a look at the Frequently Asked Questions section of there website where they go through it.