This is how you indicate a placket. Basically after you've already drawn your buttons, you need to indicate what they're buttoning into, which is a placket. So I start with right on the outside of where my buttons are, you simply do a straight line, heading all the way down the front of what this is, it's a shirt-dress.
And I usually do a little squiggle at the top and bottom to indicate where it opens up to, it's just something that's used in technical design drawings, but you don't really have to do that. If you want to have exposed stitching, you could then add like a little details like this, or just stitching, like a stitched-down placket, it's just all what you're trying to get across, or what your design sense is. And boom, there you go, that's you draw a placket.