It's a reference to Alistair Cockburn's "Ports and Adapters" pattern, also called "Hexagonal Architecture".[1] The only reason I ever heard of it is that it was mentioned in the book "Growing Object-Oriented Software, Guided by Tests", by Steve Freeman and Nat Pryce (a good book in my opinion).
[1] http://alistair.cockburn.us/Hexagonal+architecture