Understanding the Waterfall Method in API Call Management

Explain a bit how the waterfall works, especially this part here with the output. Provide a path from this action’s JSON payload to be used in the waterfall. My use case is that I’m doing three different API calls. I get different kinds of data from each API call, and I want to make sure that you’re handling the waterfall correctly - I’m getting one, then the other, then the other. I don’t understand the reason. What do we mean by this payload? It’s true that from one API call I can send data to the other API call. I really don’t get it. Now I have three HTTPs, and I’m not able to save it. I know that I could do groups and rules that if you have data in one, they move to the other. But can you explain to me a little bit what’s the difference between a group and just rules vs. this waterfall?