# Customise event field mappings

Field mappings control how Littledata maps Shopify data to destination parameters.

You can override defaults, add custom parameters, and preview the result before saving.

## Open the editor

{% stepper %}
{% step %}

### Open the destination Events tab

Start in the [Events tab](/event-editor/events-tab.md) for the destination you want to customise.
{% endstep %}

{% step %}

### Select an event

Click any event row to open its editor.
{% endstep %}

{% step %}

### Review the editor sections

The editor has three parts:

* **Event Trigger** — the Shopify event that powers the customisation.
* **Field Mappings** — the source-to-destination field relationships.
* **Event Preview** — a preview of the destination payload.

<figure><img src="/files/2b8GtgXTu7rFOe7A5TJp" alt=""><figcaption></figcaption></figure>
{% endstep %}
{% endstepper %}

## Mapping types

### Locked mappings

These mappings are required by the destination platform.

Both the source and destination are read-only.

<figure><img src="/files/pJ720fejIYsd8PPqnq2E" alt=""><figcaption></figcaption></figure>

### Editable standard mappings

These mappings use Littledata defaults.

You can change the source field, but not the destination field.

For example, GA4 maps `properties.id` to `transaction_id` by default.

<figure><img src="/files/3iUNJdJiKcAWRkGNi0zP" alt=""><figcaption></figcaption></figure>

### Custom parameters

Custom parameters let you add fields outside the default schema.

Each one has a **source dropdown** and a destination name field.

The source dropdown includes standard Shopify fields and configured [Custom Data Sources](/event-editor/custom-data-sources.md).

<figure><img src="/files/EGHqW4p0HbyDP8AtPEO7" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
Each event has a custom parameter limit. The **Add parameter** button disables at the limit.
{% endhint %}


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