Build local order book: quantity is 0, remove the price level. What is `remove` mean here?

API Spot/Margin API
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1

From Binance API Documentation

  1. If the quantity is 0, remove the price level.

Here, I am confusing.

  1. just remove the income record, don’t do anything.
  2. find previous lastUpdateId price, remove the price he set .

Which is true ?

2

If the quantity is 0, it is filled so it is no longer available from open orders.