BDC - bank transactions and source records

Unmatched bank transaction: 

Per KB, the Bank Transactions are not source records. Therefore, these transactions have no impact on the General Ledger unless they are actually added as a source record or matched to an existing source record.

However, isn't the source record created from the bank transaction when it is imported into AS.  If so, no match is ever needed. Since each bank transaction is unique, how can there be an existing source record available which is a possible match to the unmatched bank transaction record.  And from where would the possible match have been generated?