# How event topics work

To make events easy to identify, each event has a **topic**. Tasks signal their interest in specific event topics using [**subscriptions**](/core/tasks/subscriptions.md).

A topic looks like "shopify/customers/create", and it has three parts:

* The **domain** describes the source of the event. Shopify events have "shopify" as their domain, and events generated by Mechanic itself use "mechanic".
* The **subject** describes the type of resource the event describes. Events that are about customers have "customers" as their subject, and events that are about orders have "orders".
* The **verb** describes what has just occurred. Events that are about creating resources generally have "create" as their verb, and events that are about deleting resources generally have "delete".

{% hint style="info" %}
[Looking for the event topic list? Start here.](/platform/events/topics.md) Need Shopify topics? Jump to [Shopify](/platform/events/topics.md#shopify).
{% endhint %}

## User-defined topics

The User event domain is for custom, user-generated events, having any subject and verb (e.g. "user/foo/bar"). As with all events, a User event topic must use the standard three-part topic form, but only the "user/" prefix is mandatory.

Mechanic allows developers several ways to generate custom User events:

* The [Event action](/core/actions/event.md) can be used with any User event topic
* [Mechanic webhooks](/platform/webhooks.md) may be configured to generate events using any User event topic
* [Custom Shopify webhooks](/platform/shopify/custom-webhooks.md) deliver Shopify webhook events onto a User event topic, with optional filtering and payload customization. For these events, `event.topic` is the `user/...` topic and `event.shopify_topic` stores the source Shopify topic; on native Shopify deliveries, those two values match.


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