Wow, that was my exact problem!  Only, at that time, there was no input in the individual module for SE income; just gross income from farming and fishing (probably just a hobby for us CPA's!), and a couple of other rather obscure entries.

You can ignore the SE input line in the individual module; if you look carefully, you will see that the 17AC totals equal gross income on page 1, Form 1120S.  So it can't refer to net SE income directly.  (I'll admit that, when I first saw this addition to the individual input, I thought the game was finally over for all of us service businesses, but that's when I double-checked what numbers were going into 17AC.)

I've read, printed, and re-read pages 21-22 of the IRS Instructions for Schedule K-1 (Form 1120S), and it's clear that this is a catch-all box for items that need to be calculated for various obscure credits, and also a couple of items introduced by TCJA.  One of my responders made a reference to the need for this number to be available in those circumstances, and thus, its presentation on 17AC.  Like you, I'd seen nothing in that box ever before, certainly not for my garden variety Mom and Pop CPA firm, and started out assuming I'd made an input error.  I received a mixed bag of responses, most of which pointed out, in one way or another, my need to have been more specific in describing the origins of my question.  But, if only because my situation is the same as yours, or very nearly so, your question is clear to me.

My conclusion is that, for our purposes, this is an informational line that can be ignored, as we don't fit any of the situations described in detail in the IRS instructions.