> For the complete documentation index, see [llms.txt](https://docs.january.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.january.ai/client-sdks/web-sdk/concepts.md).

# Core concepts

How the SDK models clients, end users, and food data.

* [Runtime and security boundaries](/client-sdks/web-sdk/concepts/runtime-boundaries.md): where January calls can run, and which credential each place uses.
* [Client lifecycle](/client-sdks/web-sdk/concepts/client-lifecycle.md): one client per signed-in user, and when to replace it.
* [User identity and timezone](/client-sdks/web-sdk/concepts/user-context.md): the end-user ID, the timezone, and how calendar days work.
* [Food discovery and servings](/client-sdks/web-sdk/concepts/food-lifecycle.md): from search to a food with servings to a portion you can log.