The example in the reference in the piece is to have a header and a footer, where header would be open and footer closed.
As for linear types, I mean that somewhere the type system won't allow progress if you have an open type that wasn't converted to some final state. This is strictly necessary.
Ah, you don't need linear types for that. Linear types will prevent you from duplicating or discarding values, but all you need is to only allow "final state" values to be used in your API.
Also a reminder that I should read articles before commenting.
As for linear types, I mean that somewhere the type system won't allow progress if you have an open type that wasn't converted to some final state. This is strictly necessary.