Deep Entity means creating a Header + multiple Child records in one single POST call using OData service.
Instead of calling create multiple times item by item, SAP UI5 sends one payload that contains the navigation collection inside it.

Example use cases:

• Sales Order with Items

• Purchase Order with Items

• GR / Delivery with multiple line items

In UI5, you prepare the JSON payload with sub array (ex: to_Items) and call oModel.create() (V2) or create() in V4 with the same nested structure. Backend must support deep insert in metadata.

Deep Entity Insert = Faster + cleaner + transactional safe.
Most SAP enterprise apps use deep insert pattern for header / item business documents.