AppMessage transport: weather requests out, settings + readings in. Decodes the wire format and hands results to the app through handlers, so it never reaches into the UI itself.
AppMessage transport. Decodes the inbox and notifies the app through per-channel handlers, and queues the outbound weather, stock, and calendar requests through one outbox.
Which of the phone's optional features a face has, worked out from the message keys it declares. The waf build defines a HAS_MESSAGE_KEY_* for every key in the face's messageKeys, and this turns those into one switch per feature, so every file asking the same question gets the same answer.