People get these works mixed up all the time, but, at least in technical circles, the difference is important.
Let’s imagine you have a rod. The rod can be said to be extendible if it’s telescoping (like an antenna), or if it can be folded up and unfurled (like the support rods for a tent). The rod can be said to be extensible if you can put attachments onto the end to vary what the rod may be used for.
And that’s why XML stands for ‘eXtensible Markup Language’ and why EPP stands for ‘Extensible Provisioning Protocol’. It’s not just some arbitrary choice of synonym.