Only advanced accounts and enterprise accounts can apply; Some terms about API API calls are specific API services that allow you to pay, refund, search or view transaction data. API Signature A unique digital signature generated by PayPal, such as a text line or hash value. You can copy the signature from the PayPal website and use it with the API username and password * * * when making API calls. API certificate can replace digital signature; If you are using PayPal API to develop your own solution, we recommend that you choose this option. An API certificate is a file that you download from the PayPal website. The API certificate can be used to replace the API signature, and can be used together with the API username and password when making API calls. The API certificate is a text file, and your application must be able to read it every time it makes an API call. API User Name and Password The user name and password generated by PayPal can confirm your identity when you make an API call. The API username and password are different from your PayPal login name (email address) and password.