add docs

docs/binance/binance-spot/testnet_websocket-api_event-format.md

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+---
+title: "Event format | Binance Open Platform"
+source: "https://developers.binance.com/docs/binance-spot-api-docs/testnet/websocket-api/event-format"
+fetched_at: "2026-02-26T10:38:11.531Z"
+---
+[User Data Stream](https://developers.binance.com/docs/binance-spot-api-docs/testnet/user-data-stream) events for non-SBE sessions are sent as JSON in **text frames**, one event per frame.
+
+Events in [SBE sessions](https://developers.binance.com/docs/binance-spot-api-docs/faqs/sbe_faq) will be sent as **binary frames**.
+
+Please refer to [`userDataStream.subscribe`](https://developers.binance.com/docs/binance-spot-api-docs/testnet/websocket-api/user-data-stream-requests) for details on how to subscribe to User Data Stream in WebSocket API.
+
+Example of an event:
+
+```
+{    "subscriptionId": 0,    "event": {        "e": "outboundAccountPosition",        "E": 1728972148778,        "u": 1728972148778,        "B": [            {                "a": "BTC",                "f": "11818.00000000",                "l": "182.00000000"            },            {                "a": "USDT",                "f": "10580.00000000",                "l": "70.00000000"            }        ]    }}
+```
+
+Event fields:
+
+Name
+
+Type
+
+Mandatory
+
+Description
+
+`event`
+
+OBJECT
+
+YES
+
+Event payload. See [User Data Streams](https://developers.binance.com/docs/binance-spot-api-docs/testnet/user-data-stream)
+
+`subscriptionId`
+
+INT
+
+NO
+
+Identifies which subscription the event is coming from. See [User Data Stream subscriptions](https://developers.binance.com/docs/binance-spot-api-docs/testnet/websocket-api/event-format)