A plain t-shirt is usually made from a knit cotton or poly/cotton fabric.
Unattractive, knit, prairie, unadorned are synonyms for the various meanings of "plain".
I think you mean tbl (L instead of 1), which is to knit through the back loop. (Regular knit stitch is through the front loop).
The 190 is a plain stock and fore end the 290 has checkering on the fore end and stock. However the Early 290's Also have a plain stock.
Knit or body tissue
In knitting, k2tog is an abbreviation for "Knit Two Stitches Together," or simply "Knit Two Together."
the symbol is plain F.
how do you knit CR6F
It means Knit Off.
When you insert the needle into a stitch you can insert it front to back (knit-wise) or back to front (purl-wise).
When you insert the needle into a stitch you can insert it front to back (knit-wise) or back to front (purl-wise).
That probably means, "knit to marker."