You can use the one that you have, but I suggest you make specific for each client. Certificates expire, so multiple certificates give you more flexibility when you re-create, rebuild, and resubmit applications. You may also want / need to share loans with other developers or testers. With multiple certificates, you can exchange specific ones without sharing them.
But you're right, this stuff is very confusing.
My best advice: name things carefully and well.
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