Since it says invisible SEAM...it is prolly just what Jan said...the mattress seam. It can be almost invisible from the RS!
There is a side seam in this photo? Can you find it?
There is also a side seam in the middle of this photo!
There is a side seam in this pullover, too!
And a side seam in this photo, too!
Mattress seaming is the best technique I ever learned!
It is so much simpler, and easier, to do! It is worked with the right side facing you from jumpstreet...so you can see exactly what you are doing as you seam your way from bottom to top!
Glad to hear that you are conscientious about your knitting techniques! It's a good habit to get into...the fine details make or break a sweater!