Is USDD the same as USDT?

Could you please clarify for me if USDD and USDT are interchangeable or essentially the same? I've been hearing about both of them in the cryptocurrency world and am trying to understand the differences, if any, between the two. Are they both stablecoins, and if so, how do they differ in terms of their functionality, usage, or even their underlying mechanisms? Is there a specific scenario where one might be preferred over the other? Your explanation would be greatly appreciated.