**Warning**: Despite the localized name "Data source", this is NOT where you provide data - it's a widget ID selector!

**Purpose**: Specifies which widget on the same page provides entity selection events. When a user selects an entity in that widget, this connected attributes group widget refreshes to show that entity's attributes.

**Valid Widget Types**: Currently only table widgets (`cf.cplace.platform.table`) implement the required `ISingleEntitySelectionProducer` interface.

**How to Get Valid Values**: The dropdown is populated dynamically by `SingleSelectionWidgetEnumerationProvider` which finds all widgets on the same page that can provide entity selection events.

**Workflow**:
1. Ensure there is at least one table widget on the same page
2. The table widget's ID will appear in the enumeration dropdown
3. Select the table widget ID you want to connect to
4. The layout configuration field becomes visible after this is set

**Note**: The layout visibility is controlled by `showPropertyInEdit()` - layout field only shows after a selection widget is configured.