Meaning of "event time", "transaction time" fields

In the Binance Futures API, in the Websocket Market Streams messages, what is the precise meaning of the “event time” and “transaction time” fields? Which of the two should be used to compute the server-to-client latency?

I’m especially referring to the bookTicker and the order book partial update messages.
From what I understood by looking at the raw data, the event time is always after the transaction time.

I also see, in the REST endpoints, that Binance exposes a “message output time” and a “transaction time”. I’d like to understand them as well.

Hi. The transaction time T records the time that the data (e.g. account, order related) got updated and the event time E represents the time a certain data was pushed out from the server, which is the “message output time” mentioned in your post.
When calculating server-to-client latency, I would suggest to consider using event time since the transaction time can be viewed as the internal operation time at the server side.