Hi smohyee,
The "Txn" part for the fields in those tables stands for "Transaction".
In the invoice table, sales order table, and purchase order table:
TxnDate = Stands for the transaction date.
TxnMethodFlag = It is a field that allows you to flag additional data being added to the transaction for example notes that are being added to those transactions.
TxnID = It is a transaction id # that is assigned to the transaction so that the transaction can be identified in QuickBooks.
I hope that helps.
Ronen