Question
I know the K1 etc. I do not know what is meant by "lining under the sleeve?
Answer
Is this the pattern? Ready For Fall Sweater
It sounds very complicated sweater. This is the first time I have seen this too. It's just letting you know that this is the center part of the sleeve starting directly under your armpit to the beginning of the cuff. ( Similar to the seam of the sleeve you're wearing). This is the part where you'll be increasing to widen the sleeve.
Just ignore the word "lining under the sleeve" for now and keep on following the direction.
Good luck with your sweather!
Ratcha