The process continuation's service key, one per runtime layer: the platform's
app layer installs it, and a screen action or a session-establishing flow
reads it. It is the mechanism those effects use, not a
step a caller runs by hand — a caller enters the app through the platform's
launch and moves through screen actions.
A layer that installs no continuation (a caller composing the shared flows
over its own services) gets a defect when a session-establishing action runs:
there is no app process to continue.
The process continuation's service key, one per runtime layer: the platform's app layer installs it, and a screen action or a session-establishing flow reads it. It is the mechanism those effects use, not a step a caller runs by hand — a caller enters the app through the platform's launch and moves through screen actions.
A layer that installs no continuation (a caller composing the shared flows over its own services) gets a defect when a session-establishing action runs: there is no app process to continue.