In order for 3rd party software like billing systems to interface with InterWorx to perform tasks such as SiteWorx account creation, the 3rd party software must first authenticate to prove that it is indeed permitted to request actions of the InterWorx control panel system. The most common way to provide authentication is to pass what is known as an API Key. The API key will remain the same despite NodeWorx user email/password changes and there can even be a different API Key per reseller to have certain actions fall under the umbrella of a given reseller.
Note: If you re-generate your API key, you will need to replace all instances of the API key in your programs that connect to the InterWorx Control Panel service with the re-generated key.