# Concepts

> The pages in Concepts, part of Xi's Multiversion UI Lib docs.

URL: https://xichil.dev/docs/xis-mvui/v1/concepts/

- [State and refresh](https://xichil.dev/docs/xis-mvui/v1/concepts/state-and-refresh/index.md): Why widgets hold no values, when the library re-reads your suppliers, and the few things that do live inside built widgets.
- [Ids](https://xichil.dev/docs/xis-mvui/v1/concepts/ids/index.md): The rules for screen and entry ids, what they are used for, and the derived ids that dropdowns, color pickers and list editors reserve.
- [Threads](https://xichil.dev/docs/xis-mvui/v1/concepts/threads/index.md): Which of your menu code runs on the game's main thread, what may be called from other threads, and where blocking work belongs.
- [Failure isolation](https://xichil.dev/docs/xis-mvui/v1/concepts/failure-isolation/index.md): What happens when your menu code throws - which failures turn only your menu off, which are just logged, and how mistakes in builders show up.
- [Backends and versions](https://xichil.dev/docs/xis-mvui/v1/concepts/backends/index.md): How one API runs on Minecraft 1.7.10 to 26.3 - per-version backends, how the library picks one at startup, and what that means for your mod.