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

# Core Concepts

- [Authentication & User Account](https://docs.onside.io/sdk/core-concepts/authentication.md): How OnsideKit handles authentication: explicit and on-demand login, the two login flows, reading session state, and logging out.
- [The Payment Queue & Transactions](https://docs.onside.io/sdk/core-concepts/payment-queue.md): How the OnsidePaymentQueue processes transactions, why a transaction observer is required, the transaction lifecycle, and why finishing transactions is mandatory.
- [Threading & Object Lifetime](https://docs.onside.io/sdk/core-concepts/threading-and-retention.md): OnsideKit's main-actor threading model and the lifetime rules for request objects, observers, and delegates.


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