My guess is that they're calling the stitch that will be inside your seam (when you do your seaming) your seam stitch. For example, if your front was 42 stitches wide, they'd say it's 41 stitches wide plus a seam stitch.
If it relates to the K1, yo, K1 you were doing, I'd say they were having you do those increases to create an extra stitch to use for seaming. So maybe your front *was* 41 stitches, but now because of the increases, you have 42, which gives you a seam stitch.
I hope that makes sense--I'm pretty sure I understand it, but I'm not sure I'm explaining it very well!
*Dream Swatch in Madil Eden Bamboo
On the needles:
*Isabella from Spring Knitty
*Striped hoodie from Nursery Knits
*Tomato from Knitting Daily
I'm hunterjenn on Ravelry. Come find me!