API Certification
Certification is a mandatory step for some APIs before they can be utilized in the FedEx production environment. The process determines if your implementation meets a number of requirements to achieve the safe, secure and effective operation of your solution in the FedEx production environment. Differing levels of review may be required depending on the API.
Label Certification
Shipping Label Certification is a more advanced process for certain APIs where the compliance of labels generated from the test environment are evaluated by the FedEx label evaluation team.
To generate FedEx ZPL label, use our fedex plugin.
https://www.ecomplugins.com/magento2-fedex-shipping-carrier-with-rest-api