NVMe, like most computer standards, has gone through multiple revisions which add new features. That's pretty much all there is to it. If you're asking, it's probably going to make no difference to you as I believe most of the updates benefit enterprise and mobile devices (iPhones use NVMe for example). If you still want to know what 1.3 adds, here you go: https://www.anandtech.com/show/11436/nvme-13-specification-published-new-features