The centerline is just that, the boat split in half down its length, and it is not used in and of itself as a boat measurement, but as a reference point for other measurements.
Measuring parallel to the centerline at various points (e.g., at deck level, at the waterline) can give you LOA (length overall), LWL (length of waterline), and so on. LOA is defined as the straight line measurement from the foremost part of the boat to the aftermost part of the boat at deck level, measured parallel to the centerline and to the design waterline, including any integrally formed, molded, or welded components. ABYC standards carry definitions of these items.
The document titled "S-8: Boat Measurement and Weight" is the one you may be looking for. Very often, property-owners' associations contact ABYC to settle slip or mooring disputes, and this is the perfect document for that. The discussions often result in a change in the bylaws of the organization to reflect the ABYC definitions.
Brochures and online sales specs for boats often talk about terms that really don't exist in the design world but sound great at boat shows. They may quote LOA but really mean what we call "maximum length," which is the tip-to-tail measurement including any accessories installed.