# Configuration In Juju, a **configuration** is a rule or a set of rules that define the behavior of your controller, model, or application. ## Controller configuration Controller configuration affects the operation of the controller as a whole. See more: [Configure a controller](https://documentation.ubuntu.com/juju/4.0/howto/manage-controllers.md#configure-a-controller), [List of controller configuration keys](https://documentation.ubuntu.com/juju/4.0/reference/configuration/list-of-controller-configuration-keys.md#list-of-controller-configuration-keys) ## Model configuration Model configuration affects behavior of a model, including the `controller` model. See more: [Configure a model](https://documentation.ubuntu.com/juju/4.0/howto/manage-models.md#configure-a-model), [List of model configuration keys](https://documentation.ubuntu.com/juju/4.0/reference/configuration/list-of-model-configuration-keys.md#list-of-model-configuration-keys) ## Application configuration Application configuration affects the behavior of an application. Application configuration keys are generally application-specific. Depending on what the charm author has decided, they can be used to allow the charm user to make certain decisions about the application, for example, the server port on which the application should be available, the resource profile, the DNS name, etc. See examples: [Charmhub | `mysql` > Configurations](https://charmhub.io/mysql/configure#cluster-name), [Charmhub | `traefik-k8s` > Configurations](https://charmhub.io/traefik-k8s/configure), etc. See more: [Configure an application](https://documentation.ubuntu.com/juju/4.0/howto/manage-applications.md#configure-an-application) However, there is also a generic key, `trust`, that can be changed via `juju trust`. See more: [Trust an application with a credential](https://documentation.ubuntu.com/juju/4.0/howto/manage-applications.md#trust-an-application-with-a-credential)