Commonly confused terms
Pairs people mix up all the time — side by side, with the tell that separates them.
Overlay vs Split
Original Assignee vs Current Service Provider
NXX (prefix) vs Thousands-Block
Caller ID vs CNAM
Area Code vs Rate Center
Overlay
Adding a new area code over the same geography as an existing one, so both serve the region and ten-digit dialing becomes required.
An overlay does not change anyone's existing number. It is often confused with a split, which does. The tell: after a split, some people get a new area code; after an overlay, nobody does.
Split
The older relief method that divides an area code's geography, forcing part of the region onto a new area code.
"Split" and "overlay" are not interchangeable. A split reassigns part of the map to a new code and forces renumbering; an overlay layers a new code over the same map and forces only ten-digit dialing.
How to tell them apart. The tell: after a split, some people get a new area code. After an overlay, nobody does — only dialing changes.