This is something that started as a way to get data back from Actors in non-actor code (for example, web services). I've never cared for the blocking nature of Reply Msgs and the only other built-in option for getting back data is to make everything an Actor, which is not always an option. Promises solve both of those issues.
The basic idea is an enforced single-writer, many-reader cross thread datatype. In the current implementation, they are not much more than a locked-down, single element queue but you can actually do some pretty cool stuff with just that.
In the message sender, we create the promise and return it. The idea here is that the Actor owns the promise and will fulfill it. This gives us very low coupling for free.
Wait on Promise will wait for the Promise to be fulfilled. It is a malleable VI so that a Default Value (for timeout) and Type can be wired. Using the timeout lets us do other things while waiting for data.
Inside the Actor, we can set the value with Fulfill Promise. Remember, once the Promise is set, that's it, no changing it again. In fact, Fulfill Promise will error out if called twice on the same promise.
Something else really cool we can do is fulfill a promise with another promise. This may seem pedantic at first but it can help keep your code cohesive by passing the responsibility of fulfilling a Promise to another process. For example, if you have a message broker that just forwards a message, you can have the message broker fulfill it's promise to the caller with a promise from the callee.
We can also reject a promise with an error message that will be returned by any Wait on Promise that tries to read it. Rejecting a promise does fulfill the promise so you still cannot set the value later and you cannot reject a promise again.
Finally, we have Destroy Promise. It does exactly what it says on the tin.
I'd love to hit some more of the design points from https://promisesaplus.com/ (mainly adding Then callbacks) but I figure it's at a pretty good spot to share and get some feedback. I'd also like to try to figure out network communication at some point but that's probably a ways away (without some help anyway ).