Fill mode is for using a Pattern someone else built, or one you built last month. You see the Pattern, change the free Stitches and knit.
What you can change
Only free Stitches: the headline, the photo, the caption. The locked ones are drawn with a dashed outline and do not respond, so you cannot break the brand by accident.
Today, through your model
Ask your model to fill the slots of a Pattern. It writes by slot name, and anything locked is refused and reported back. See Using your model.