From the pattern:

The pattern features a slipped stitch heel cleverly disguised as 2×2 ribbing. Topped with a picot bind-off, the ankles are knit in a dropped stitch lace pattern that is easy to remember, fast to work. Twisting the stitches around the dropped stitch circles helps to keep the lace open. The ribbing is quite stretchy and will accommodate most leg sizes.

Instructions are for women's size 7/8. Changes for sizes [5/6, 9/10] are given in brackets. Adjust if necessary. The stitch pattern requires a multiple of 8 stitches and 12 rows.