It’s hard to fully understand all this certificate stuff for IOS development and it’s implications.
One piece of info I can not find in the docs or via google: In order to join multiple developer programs (when working on software for different clients), should I create a unique developer certificate for each, or can I use the one I already have?
Is there an advantage/disadvantage in either way?
You can use the one you have, but I suggest you make specific ones for each client. Certificates expire, so multiple certs gives you more flexibility when re-creating, re-building, and re-submitting apps. You also may want/need to share creds with other devs or testers. With multiple certificates, you can share specific ones without sharing all of them.
But you’re right, this stuff is very confusing.
My best advice: name things carefully and well.