cme e-mini s&p500 time&sales - 期貨

各位老手大家好

想請教一個問題就是我在看CME E-mini S&P500 time and sales的資料
連結在此http://tinyurl.com/dxstkwd

在價格數字後面會有AB的英文字母
看他網頁的解釋

Some prices are followed by an "A" after the price indicating an
offer/ask price which occurred at or below the previous last.
A "B" after the price indicates a bid which occurred at or above
the previous last.

不太懂的是成交價就成交價，為啥有賣價格比前一筆低就標a，買價格比前一筆高就b
並且他們的交易量都是0
看不太懂這個數據的意思，不知道有沒有前輩可以教導一下，感謝！

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