Inject Node: "flow." and "global." variables?

The Inject Node provides ways to inject strings, numbers, booleans, timestamp, and also “flow.” and “global.”. What’s the “flow.” and “global.” do? How are people using it?

These are such common terms I’m having trouble finding search results that talk specifically about it.

flow and global variables allow you to pass variable values between nodes either just within a flow or any nodes on any tab.

They are explained here.

But you can also set them using the change node

flow. like global variable. It exists at flow.
When you use http-in to get request and set some flow.
When another one request comes in it will see the same flow. variable.
Be careful to use flow. or global.