What Is Granularity?

When I joined PayPal back in 2014, I was given slash payment. And said, okay, six attributes defines my payment and I can start designing the basic building block of that particular payment. Then once you have your granularity, then you come into intersections. How are these different objects or offerings of APIs connected with each other? We are probably defining a flow or a certain use case which always calls API 1, then API 2, then API 3, and API 1 again - but there is a lot more that needs to happen.