Can you provide some context? Where do you see this, exactly?
I think I remember something fairly similar in the Whitney instructions, although it differs a little bit. There’s the first seam at 1/2”, and then second seam at 3/8”, and then there was the third seam as a zig-zag on the edge of the seam allowance. The zig-zag is broken on my (borrowed!) machine, so I ended up just triple stitching all my seams with a straight stitch.
Basically, look at your seam allowance—imagine three parallel lines running alongside the edge you’re sewing at different distances. That’s basically what you’re looking at.
Unless it’s different in your context, in which case I could be totally wrong, but that’s my best guess based on your question.
—Peter