Prior to the release of UFT 11.5 the GUI and API test automation tools could be purchased and licensed separately. They were known as QuickTest Pro and Service Test, respectively. With UFT 11.5 the two tools have been combined into one product. If you have UFT 11.5 and have licenses for both GUI and API test automation then the documentation for the API test automation capabilities should be contained in the help files of the product.
If you are working with a version older than UFT 11.5, then look for documentation for Service Test. 11.20 was the latest version of that stand alone product.