> 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/ios-sdk/concepts.md).

# Core concepts

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

* [Client lifecycle](/client-sdks/ios-sdk/concepts/client-lifecycle.md): when to create, keep, and replace a `JanuaryClient`.
* [User identity and timezone](/client-sdks/ios-sdk/concepts/user-identity-and-timezone.md): what the end-user ID and timezone control, and how days are counted.
* [Food details and portions](/client-sdks/ios-sdk/concepts/food-hydration-and-portions.md): from a search result to a portion you can log.