> For the complete documentation index, see [llms.txt](https://docs.cybus.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.cybus.io/2-6-1/data-flows/service-commissioning-files/definitions.md).

# Definitions

Define reusable value blocks in a service commissioning file and reference them with the !ref or !merge functions.

Definitions allow you to reference blocks from within a service commissioning file. While they share similarities with parameters, definitions are designed for internal use only.

You can define values of any data type, including object types. These values can be referenced using the `!ref` or `!merge` functions.

## Example

In this example, we create a definition for default Modbus endpoint settings that we want to reference.

{% code lineNumbers="true" %}

```yaml
modbusEndpointDefaults:
  connectionId: !ref modbusConnection
  operation: subscribe
  interval: 1000
  address:
    fc: 3
    length: 2
```

{% endcode %}

When you reference this definition using `!ref modbusEndpointDefaults`, you get the following value:

{% code lineNumbers="true" %}

```yaml
connectionId: !ref modbusConnection
  operation: subscribe
  interval: 1000
  address:
    fc: 3
    length: 2
```

{% endcode %}


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