They're mostly interchangeable. In a business setting, the client would mostly refer to an individual, while the customer would (in my experience) refer to the company. For example:
Today I spoke with the client, and we decided we would proceed as planned.
We visited the customer's main factory today and I met with Steve John and Allan.
It's mostly about what feels right, neither is incorrect and they are commonly swapped about to the speakers preference.
Thank you everybody. great question and answers
Thank you for your answer. Now it became more understandable.
Thanks for your explanation.