> For the complete documentation index, see [llms.txt](https://docs.umbraco.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.umbraco.com/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/terminology.md). # Terminology A list of some of the key concepts with working the Umbraco Backoffice. Understanding certain key concepts is essential when customizing the backoffice. These terminologies can help you decode the purpose of code effectively: * **Repository:** An API that enables communication with a server. * **State:** A reactive container holding data, when data is changed, all its Observables will be notified. You can read more about states and observables in the [States](/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/states.md) article. * **Observable:** An observable is the hook for others to subscribe to the data of a State. * **Observe:** Observe describes what we do when subscribing to an Observable. * **Context-API:** The name used to serve APIs (instances/classes) for a certain context in the DOM. An API that is served via the Context-API is called a Context. You can read more about this in the [Context API](/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/context-api.md) article. * **Context Provider:** Enables providing a class instance as a Context API. * **Context Consumer:** Enables the consumption of a Context API. * **Controller:** An abstract term for a thing that hooks into the lifecycle of an element. Many things in our system are Controllers. * **Umbraco Controller:** Enables hosting controllers. Additionally, it provides a few shortcut methods for initializing core Umbraco Controllers. You can read more about this in the [Umbraco Controllers](/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/umbraco-controller.md) article. * **Controller Host:** A class that can host controllers. * **Controller Host Element:** The element that can host controllers. * **Umbraco Element:** The `UmbLitElement` or `UmbElementMixin` enables hosting controllers. Additionally, it provides a few shortcut methods for initializing core Umbraco Controllers. You can read more about this in the [Umbraco Element](/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/umbraco-element.md) article. Read more about how to get started with extending the backoffice in the [Backoffice Setup](/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/extending-overview.md) article. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://docs.umbraco.com/umbraco-cms/17.latest/extend-your-project/backoffice-extensions/foundation/terminology.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.