Both "Butterschmalz" and "Butterreinfett", have similar meaning in Leo. I can't identify the difference.
I believe "Butterreinfett" is the technical term with specific meaning while "Butterschmalz" is a common term.
Both denote a type of clarified butter where water, milk protein and lactose have been removed by (among others) heating and centrifugation. "Butterreinfett" has at least 99.8% of fat.
The common term, "Butterschmalz", might have been coined in order to associate it with Schmalz=lard, (which it is not).
You are more likely to find "Butterreinfett" used on a list of ingredients (e.g. of chocolate) and "Butterschmalz" more likely find as a product name in a shop.