Opaque user-provided value, which will be passed to the other callbacks. The close callback will be called to release the cookie. It is not interpreted by mpv. It doesn't even need to be a valid pointer.
Callbacks set by the user in the mpv_stream_cb_open_ro_fn callback. Some of them are optional, and can be left unset.
See mpv_stream_cb_open_ro_fn callback.