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Unstable release is being listed as UST in
Yet we see it having hashtag #MTGUN3 at official product page https://magic.wizards.com/en/products/unstable
Will it be UST or UN3 eventually?
The set code is UST. It's already official, and you can see it printed on the bottom left of your card, in this case on Adorable Kitten:
The set code is always present there.
The hashtag may be #MTGUN3, but it's still UST for a set code.
The Official set code is UST, and can be found in the bottom left corner of all cards from the set.
As for why some tweets from Official sources use the #MTGUN3, #MTGUN3 was already a popular hashtag and wizard's did not want to split the marketing effort between two hashtags.
This question was actually covered on Mark Rosewater's Blog:
howdidigethereimnotgoodwithmagic asked: Why was the Hashtag for Unstable #MTGUN3 when the set code on the cards is UST?
MR: Because we didn’t want half the people using #mtgust and half using #mtgun3. In other words, we went with the flow.
