https://www.irs.gov/pub/irs-pdf/i1120s.pdf

Bottom of page 23 (it's actually the K-1 instructions, not the 1120S page 3 instructions).  It's tied to the passive activity reporting requirements, but in practice, I don't see how you can determine QBI at the activity level without this information being split out and reported.

Edit:  Oops, meant to paste the text:

If items of income, loss, deduction, or
credit from more than one activity
(determined for purposes of the passive
activity loss and credit limitations) are
reported on Schedule K-1, the corporation
must provide information separately for
each activity to its shareholders.