It means state is passed in as a parameter to the state machine interpreter - as opposed to being contained in the interpreter object. This is helpful when building pure functional systems, where state is passed in from some external data store, like in Redux reducers.

Thank you - this is correct (the interpreter is impure). It also makes it possible to create your own custom interpreters.

Also, your work has been a huge influence on XState, and I can't thank you enough for SCION!

Thanks!

This took me a while to figure out, but it’s incredibly powerful once you get it. As others have suggested, I was working on a Redux-based interpreter.

I would call it "external state". Instead of having the state tightly coupled to the state machine it can be persisted somewhere else.