How do I account for various line items in IT3b Discovery investment certificate?

I receive two tax certificates from Discovery for the same investment product. One is an IT3b, the other an IT3c. While the IT3c shows a source code of 4250, the IT3b does not show source codes. It shows "Gross foreign dividends", "Local interest voluntary", "Gross foreign interest" & "Local dividends (exempt)". It also shows withholding tax on 3 of them, and a "Net total".

When declaring my local interest earned, should I add just the "Local interest voluntary" amount from Discovery to my bank's IT3b amount for local interest total?

Yes, you can add the Discovery interest to your bank's IT3b interest and declare the total under "local interest" on your Tax Return.