I have misspoken here. I had forgotten that the beverage made from the same plant in the Jamaican region is known as sorrel. Sorrel the plant is unrelated, but has a tart taste, and the leaves can have a red colour.

Roselle

Sorrel

The calyx that grows from the flower (kinda like a fruit) is the part that is used for the beverage. I don’t believe all hibiscus produce the fruit, just this varietal. Roselle

I'm guessing flower heads, but that's a major omission from the linked recipe.

Checking a little, looks like the dried flower sepals are commonly sold in Mexican markets.