Selects which channel or network the request acts on.

This applies to every endpoint in this API, not only GET /episodes. Its requirement
depends on what the access token's resource owner is:

• Channel token — ignored. The token already identifies the channel.
• Network token — optional. Omit it to act on the network itself; supply the subdomain
  of one of the network's channels to act on that channel instead. An unrecognised subdomain
  returns 400.
• User or superapp token — required. The token is not bound to an entity, so the
  target must be named on every request. Omitting it returns 400; naming an entity the user
  does not manage returns 403.

The X-CHANNEL-SUBDOMAIN header takes precedence over this parameter. A legacy subdomain
query parameter is also still accepted as a fallback.

A subscriber's e-mail address or Supercast uuid. Used to personalise the response for that listener. When omitted or unresolvable, the per-viewer fields are returned empty rather than erroring.

Header form of channel_subdomain, and the preferred one — it takes precedence over both the channel_subdomain and legacy subdomain query parameters. See channel_subdomain for when a value is required.