TerraUSD (UST) is an algorithmic stablecoin pegged to the US Dollar running on the Terra (LUNA), Ethereum (ETH), and other blockchains. Avalanche now has native UST.
Stablecoins can be tricky in how they are seen and moved, as many times, the Token name looks the same but the network it is native to is not notated in its name. So UST can exist on the Ethereum network as UST, also on Terra as UST, but also on Avalanche as UST. So one may think that it can be sent across different blockchains directly but it cannot. It must be bridged.
UST.axl is Axelar wrapped UST that was bridged over from the Terra Network using the Satellite bridge. This is different than Avalanche native UST. To convert UST.axl into Avalanche native UST, it must be swapped on a DEX, such as TraderJoe and Pangolin.