My Pattern isn't Making Sense

My pattern reads:
Beg with a purl row, work 1 (1½-1½-2½) ins [2.5 (4-4-6) cm]
in stocking st from cast off sts, ending with RS facing for
next row.
Where am I knitting into? Do I make more stitches from the cast off ones? or am I knitting everything else?

Can you give us some context? What comes before this?

You’re knitting everything else, the stitches that were left over after you cast off some of them. Or did you cast off all of them?

Here is what come’s before and includes the questioned pattern:

Leg Openings: Next row: (RS). K5 (5-7-11). Cast off
next 4 (6-6-10) sts. K36 (46-56-80) (including st on needle
after cast off). Cast off next 4 (6-6-10) sts. Knit to end of row.

Note: All Leg sections are worked at the same time using
separate balls of yarn for each section.

Beg with a purl row, work 1 (1½-1½-2½) ins [2.5 (4-4-6) cm]
in stocking st from cast off sts, ending with RS facing for
next row.

Okay, beginning with where your yarn is at the end of the last cast off sts, start knitting in stockinette.

But, there were stitches after the cast-off stitches. Do I just start knitting in stockinette there, and end up joining the stitches around the cast-off sections?

I think that you would attach different skeins of yarn to each side. Does that answer your question?

There was another thread [COLOR=“Blue”]here[/COLOR] with similar questions on same (?) pattern - perhaps it would help also.