Does anybody know what value "timestamp" in orderbook POST actually means?

The response is structured as follows:

• bids (array)
• price (price)
• amount (decimal)
• timestamp (time)
• asks (array)
• price (price)
• amount (decimal)
• timestamp (time)

documentation: Bitfinex API

Does it say when the corresponding limit order entered the orderbook? Assuming that this is true, what if there are two limit orders at the same price level each of which entered the orderbook at different time? Will there be two orders at that price level or will it be groupped to one order with the latest (more recent) timestamp?