In my own experience, I've seen them as distinct groups from customer support both at the companies I've worked for and companies I've worked with. I view it more as an alias to account rep, but sometimes people account reps will delegate to.

Customer support will file a ticket for me when something isn't working. Maybe help if something on their script matches existing known issues. Customer success will proactively see that I'm not getting the expected value from my already committed spending and utilization and reach out to make sure everything is configured right so that I renew next year.