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## In Rasa, your `domain` defines the universe in which your assistant operates.
Specifically, it lists:

- The `responses` that can be used as templated messages to send to a user.
- The custom `actions` that can be predicted by dialogue policies.
- The `slots` that act as your assistant's memory throughout a conversation.
- Session configuration parameters including inactivity timeout.

If you are building an NLU-based assistant, refer to the [Domain documentation](https://legacy-docs-oss.rasa.com/docs/rasa/domain) to see how intents, entities, slot mappings, and slot featurization can be configured in your domain.

## Multiple Domain Files

The domain can be defined as a single YAML file or split across multiple files in a directory. When split across multiple files, the domain contents will be read and automatically merged together. You can also manage your responses, slots, custom actions in [Rasa Studio](/content/docs/studio/intro/index.html).

Using the [command line interface](/content/docs/reference/api/command-line-interface/#rasa-train/index.html), you can train a model with split domain files by running:

```bash
rasa train --domain path_to_domain_directory
```

## Responses

Responses are templated messages that your assistant can send to your user. Responses can contain rich content like buttons, images, and custom json payloads. Every response is also an [action](/content/docs/reference/config/domain/#actions/index.html), meaning that it can be used directly in an `action` step in a flow. Responses can be defined directly in the domain file under the `responses` key. For more information on responses and how to define them, see [Responses](/content/docs/reference/primitives/responses/index.html).

## Actions

[Actions](/content/docs/reference/primitives/actions/index.html) are the things your bot can do. For example, an action could:

- respond to a user,
- make an external API call,
- query a database, or
- just about anything!

All [custom actions](/content/docs/reference/primitives/custom-actions/index.html) should be listed in your domain. Rasa also has [default actions](/content/docs/reference/primitives/default-actions/index.html) which you do not need to list in your domain.

## Slots

Slots are your assistant's memory. They act as a key-value store which can be used to store information the user provided (e.g. their home city) as well as information gathered about the outside world (e.g. the result of a database query). Check out the [Slots reference](/content/docs/reference/primitives/slots/index.html) for more information.

## Session configuration

A conversation session represents the dialogue between the assistant and the user. Conversation sessions can begin in three ways:

1. the user begins the conversation with the assistant,
2. the user sends their first message after a configurable period of inactivity, or
3. a manual session start is triggered with the `/session_start` intent message.

You can define the period of inactivity after which a new conversation session is triggered in the domain under the `session_config` key.

Available parameters are:

- `session_expiration_time` defines the time of inactivity in minutes after which a new session will begin.
- `carry_over_slots_to_new_session` determines whether existing set slots should be carried over to new sessions.
- `start_session_after_expiry` (new in 3.16, default `true`) determines whether `action_session_start` runs when the next user message arrives after the session timer fires. When `false`, slots are preserved without a session boundary. See [Session Timer](/content/docs/reference/config/session-management/session-timer/#start_session_after_expiry/index.html).

The default session configuration looks as follows:

```yaml
session_config:
  session_expiration_time: 60  # value in minutes, 0 means infinitely long
  carry_over_slots_to_new_session: true  # set to false to forget slots between sessions
  start_session_after_expiry: true  # new in 3.16; must be false when a2a_server is enabled
```

This means that if a user sends their first message after 60 minutes of inactivity, a new conversation session is triggered, and that any existing slots are carried over into the new session. Setting the value of `session_expiration_time` to `0` means that sessions will not end (note that the `action_session_start` action will still be triggered at the very beginning of conversations).

Required for A2A sub-agent mode

When `a2a_server` is configured in `endpoints.yml`, set `start_session_after_expiry: false`. Resumed orchestrator `contextId` values map to the same Rasa `sender_id`; auto session restart would reset slots silently.

See [A2A Server](/content/docs/reference/integrations/a2a-server/#session-configuration/index.html).

note

A session start triggers the default action `action_session_start`. Its default implementation moves all existing slots into the new session. Note that all conversations begin with an `action_session_start`. Overriding this action could for instance be used to initialize the tracker with slots from an external API call, or to start the conversation with a bot message. The docs on [Customizing the session start action](/content/docs/reference/primitives/default-actions/#customization/index.html) shows you how to do that.

## Select which actions should receive domain

New in 3.4.3

You can control if an action should receive a domain or not. To do this you must first enable selective domain in you endpoint configuration for `action_endpoint` in `endpoints.yml`.

endpoints.yml

```yaml
action_endpoint:
 url: "http://localhost:5055/webhook" # URL to your action server
 enable_selective_domain: true
```

**After selective domain for custom actions is enabled, domain will be sent only to those custom actions which have specifically stated that they need it.** To specify if an action needs the domain add `{send_domain: true}` to custom action in the list of actions in `domain.yml`:

domain.yml

```yaml
actions:
  - action_hello_world: {send_domain: True} # will receive domain
  - action_calculate_mass_of_sun # will not receive domain
  - validate_my_form # will receive domain
```

- [Multiple Domain Files](/content/docs/reference/config/domain/#multiple-domain-files/index.html)
- [Responses](/content/docs/reference/config/domain/#responses/index.html)
- [Actions](/content/docs/reference/config/domain/#actions/index.html)
- [Slots](/content/docs/reference/config/domain/#slots/index.html)
- [Session configuration](/content/docs/reference/config/domain/#session-configuration/index.html)
- [Select which actions should receive domain](/content/docs/reference/config/domain/#select-which-actions-should-receive-domain/index.html)