Deciphering a knitting instruction by: Ratchadawan
Garter sts is when you knit every row. ST st or stockinette sts is when you knit one row and purl one row alternately.
You can tell the different between these two stitches by the way they form. Garter stitch produce ridges and waves on both right and wrong side. Stockinette stitch has a smooth surface on the right side and wavy on the wrong side.
to work your pattern of 15 garter sts, 90 sts ST st, l5 garter sts would be like this.
Row 1 (right side): Knit all 120 sts. Row 2 (Wrong side): K 15, p 90, k15.