It is just a way to define a lambda without the import of Functions and the need for all the awkward stuff one needs to use to define a lambda. The Rules DSL is smart enough to figure out what the lambda is (Function or Procedure) and what the types of the arguments and return value is on its own when defining a lambda like this.
Same as any other lambda. The lambda definition I provided is identical to Alexandre’s (with a couple of minor changes to the body).
I don’t use lambda in design patterns typically. I’ve not gone back to update Reusable Functions: A simple lambda example with copious notes yet.