Tassie Cups is a term that can mean both the cups (indentations) in a muffin tin, and, the small tart shells made of pastry.
If a recipe says “Buy (or make) tassie cups”, it means pastry shells.
Otherwise, the recipe will specify something such as “foil tassie cups.”
Language Notes
Tassie is a word in Scottish that actually means “cup”, as in from the French word for cup or glass, “tasse.”