When you download transactions from your financial institution and select the Show Register checkbox while viewing a QuickStatement (a list of transactions that have taken place since your last download, and any transactions not matched from previous downloads), QuickBooks tries to match transactions from the statement with those in your register.
The matching process consists of several stages, using the algorithm described here. All of the following criteria must be met for QuickBooks to consider transactions matched.
Before the matching process
Before beginning the matching process, QuickBooks identifies all register transactions dated more than 90 days before the system date of the computer on which QuickBooks is installed, or more than 60 days before the earliest date of all the downloaded transactions. These identified register transactions will not be considered for matching. Transactions that have already been reconciled are also discarded from matching consideration.
Primary matching process
Secondary matching process
Supplementary matching process
QuickBooks compares the register transaction examined during the secondary matching process against each unmatched transaction in the QuickStatement. If it finds a better pair than was found during the secondary matching process, then it uses the new match instead. Otherwise, QuickBooks creates a match between the current register transaction and the downloaded transaction compared during the secondary matching process.