Which cards are legal in MTG Arena's pauper format?

MTG Arena doesn't seem to have information available on which cards are legal in its Pauper format. Does this format only allow cards which have had Common rarity in MTG Arena itself? Or is it simply Standard Pauper? I'm not sure if that's the same thing or not—I just started playing Arena recently and am not sure if there has been a set rotation since it began. I have noticed some cards being played at Uncommon rarity but I think they all have Common printings from another set which are still Standard-legal.

Jon N points out that with the Arena Deckbuilder's Advanced filter set to only show commons, some uncommons will nonetheless appear.

Although there is no official format definition for Arena, Pauper as an older format has always been defined as: "cards with a common printing in allowed sets". So in Arena, this could be translated as:

Any card which has a common printing in Standard (i.e. sets which are legal in Arena), no matter if the specific printing of the card is in other rarity.

Having said that, I don't think currently there is a card with printings at different rarity in Standard sets.