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While viewing any register you can use the Search field in top, right corner of the register to search/filter the register being viewed. You can search all visible fields (default) or limit your search to specific fields such as Category, Payee, Notes, etc.
You can also filter your register to a specific date range, transaction type, or transaction status. See also the help topic Finding transactions.
Once you can see the transactions you'd like to export, you're ready to go. (If you apply no filters, you can export all transactions in the register.)
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