I've seen all kinds of things from across editions of D&D even discussing what the tiefling tail can and cannot do.
Some books suggest Tiefling's tail is vestigial. Some say say they're not vestigial, but aren't prehensile. Some say they are partially prehensile. Some say they are fully prehensile. The topic seems to be hotly debated on some areas of Reddit. These discussions are all very interesting I'm sure, but Arelith is its own beast.
So in the context of D&D lore of an era appropriate to Arelith, what is a Tiefling's capable of doing? Balance? Holding small objects? Giving an unruly friend a thwap on the head? Rocking the ranger and hanging from a tree possum style? Bench pressing the current Cordor legal code?
Do the staff have any thoughts on what these capabilities cannot be, or does this fall under "be reasonable?"