The "store" is purely data that is on the client. What data you put in the store is up to you. It may be a mixture of domain data from the server ("what items do I have in cache"), application-specific data ("the current selected item is #42"), and UI state ("there is a modal that is visible, of type ItemEditor"). It's certainly possible to put every single bit of data in your app into Redux, but there are definitely reasons to want to keep some of it in local component state. The classic example is "a dropdown is open" - other components probably wouldn't care about that, and it's not something you'd want to persist or toggle during time-traveling. It's up to you as a developer to decide what state should live where.
There's a couple recent articles that cover this topic: https://medium.freecodecamp.com/where-do-i-belong-a-guide-to... and http://jamesknelson.com/5-types-react-application-state/ . The Redux FAQ also discusses this: http://redux.js.org/docs/FAQ.html#organizing-state-only-redu... .