Only the assist kicker cost (or part of that cost) can be paid by another player
Its rarities cover Super Rare (SR), Collector's Rare (CR), Ultra Rare, Secret Rare, and others
There are two deckbuilding details I want to mention still: Some decks run as little of the "U.A." monsters as possible, potentially running only one copy of every high-level monster because they are easily searchable and accessable
10 | Pyroblast Price: $7.75 Seeing strong play in Pauper, Premodern, Legacy, Vintage, and Commander, Pyroblast is a true multiformat staple
The standard stock in the open market is usually close but not quite in thickness, stiffness, or finish
In short, it's a format that the most played decks in the metagame are not allowed