What is a key characteristic of add-on codes?

Add-on codes are specifically designed to provide additional information about a procedure or service that is performed in conjunction with a primary procedure code. These codes are not meant to stand alone and are rarely reported without a primary procedure. This characteristic reflects their function in coding systems, where they enhance the specificity of the primary code by detailing additional, related services or procedures that occur simultaneously or as part of a broader treatment plan.

In contrast, the other answers highlight characteristics that do not align with the nature of add-on codes. For instance, add-on codes cannot be used independently, as they are inherently linked to a primary procedure. Additionally, add-on codes are not restricted to non-surgical procedures; they can apply to both surgical and non-surgical contexts. Lastly, asserting that add-on codes are applicable to all types of medical services is misleading, as their use is specifically regulated within coding guidelines that dictate their appropriate pairing with primary codes. Thus, the uniqueness of add-on codes and their dependence on primary codes solidifies the correctness of the selected answer.