Difference between a geographic split and an overlay plan
Geographic split - A new area code created by designating new geographic boundaries. In most cases, this means taking an existing area code and dividing the geographic territory into two areas. The residents and businesses living in the geographic area that gets the new area code will have their numbers changed to the new area code.
Overlay - New area code given to customers getting new lines. For example, a new business owner may have a different area code from his/her next door neighbors. No changes for existing phone numbers, but all calls (local and long distance) must be dialed using 10 or 11 digits.