This can be used on a method to indicate that it is used for "streaming", like:

    write @0 (bytes :Data) -> stream;

A "streaming" method is one which is expected to be called many times to transmit an ordered stream of items. For best throughput, it is often necessary to make multiple overlapping calls, so as not to wait for a round trip for every item. However, to avoid excess buffering, it may be necessary to apply backpressure by having the client limit the total number of overlapping calls. This logic is difficult to get right at the application level, so making it a language feature gives us the opportunity to implement it in the RPC layer.

We can, however, do it in a way that is backwards-compatible with implementations that don't support it. The above declaration is equivalent to:

    write @0 (bytes :Data) -> import "/capnp/stream.capnp".StreamResult;

RPC implementations that don't explicitly support streaming can thus instead leave it up to the application to handle.