I wondered why people keep typing that. I've just looked it up and it brought me here. Are we supposed to add the households to our libraries and then add the #mccc?
What you saw in the comment section is the wrong use of #mccc_include. To use it for being used as a tray sim (this is for preventing randomly generated sims), you have to type this tag on the sim description in your own library, not the comment section.
I can't explain it clearly, but if you want to know more, you might search for the tag, or words "Import Tray Sim".
I had extra searching today, and found this might be useful to understand.
https://thesimstree.com/en/blog/the-sims-tips/what-is-the-mccc-include-tag-and-how-to-change-npcs-in-sims-4.html
