Hey Mary Lee.
Connecting the open items with the payments is done via Note to payee functionality. You can chech that in Financial Accoutning (New) – Contract Accounts Receivable and Payable – Business Transactions – Payments – Processing Incoming and Outgoing Payments – Define Interpretation Rules for Note to Payee (and the following programs in IMG, where you can test your setting).
Using that functionality, you can define how will you "read" the note to payee field from the electronic bank statement (you have that field in the MT940 file format, if youre using it; if youre using the local country file convertor, then you have to check which fields get written in the needed field) and how will you extract the data to identify the open item. For example, you can use the document number, or the combination of contract account and reference document number, or any other solution that fits your business scenario.
Marko