The subscriber's Supercast uuid, e-mail address, or customer_id (the payment gateway's customer identifier). Matched in that order of preference: e-mail, then uuid, then customer_id.

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.

Network tokens only. When present, the subscriber lookup searches across the network and all of its channels rather than the network's own subscribers alone. Any non-blank value enables it — only the presence of the key is checked. Ignored for channel tokens.

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.