Announcing UNISTR and || operator in Azure SQL Database – preview

Is this a binary operator or more like syntax sugar for CONCAT?

On the "at least one input is a character string" is there any requirement that this be in the first two arguments?

i.e. would the following work? 

SELECT 1 || 2 || 3 || 'Foo'

(Edit: I tested the above and it does fail!)