QuickBooks imports bank transactions in a file format called OFX. Each transaction in an OFX file has a unique transaction id. Below is an example - in this case the transaction ID is the number following <FITID> or [removed]. The numbers _must_ be unique - they can never be used in the same bank account for two different transactions. If they are that's a bank error (Collect $200! No, just kidding!)
QuickBooks keeps track of the transaction ids that have been imported and what you've done with them. For instance if you delete a transaction from the download transaction window QB will track that FITID number. Next time you pull transactions from the bank that same transaction may get pulled down again. If it is, QB will see that you already marked it "deleted" and it won't show up in the list of downloaded transactions.
<STMTTRN>
<TRNTYPE>CHECK
<DTPOSTED>[removed]00[0:GMT]
<TRNAMT>-2853.00
<FITID>[removed]
<CHECKNUM>3070
<NAME>CHECK 3070
</STMTTRN>
