Many companies already use a third party API integration software application to run their business operations, but find that the third party API software is lacking features or customization that is needed to fit perfectly. While they are unable to abandon the current software systems due to financial reasons or logistics, they still have the option of writing a custom application that integrates with the third party API software. A third party API integration interface is a document, supplied by 3rd party software vendors, that contains all of the technical details required for programmers to integrate software to a custom application. If your software vendor is able to supply the API documentation for the software system, then you have an opportunity to integrate to a custom software application that is specialized for your needs.