Purl Right Side?

I am new to knitting, so forgive me if this is obvious to most of you. I have instructions that say for a particular row to “Purl on RS, Knit on WS”. Doesn’t one usually knit the RS and Purl the WS? If so, why would a pattern tell me to do the opposite?

Thanks

Because this is a reverse stockinette stitch? Or if only for one row there’s a purl ridge that shows on the RS.

You think maybe the purl on RS is in effect for a decorative ridge?

Another reason I am confused about this instruction to “Purl on RS” is because the step-by-step text instructions for the row say simply to “Knit”, but the diagram instructions for the very same row indicate to “Purl on RS, Knit on WS”. I don’t know how to follow the instructions that seem to me to say to do two different things for the row. I am on the RS starting this row. Do I knit, or do I purl?

Thank you for your help.

Without knowing more about the pattern, I can’t say. Do you have a link to it or to pictures?