The event is emitted on ACPI OSPM Status Indication events.

ACPI standard documentation describes the method as:

This object is an optional control method that is invoked by OSPM to
indicate processing status to the platform. During device ejection,
device hot add, or other event processing, OSPM may need to perform
specific handshaking with the platform. OSPM may also need to indicate
to the platform its inability to complete a requested operation; for
example, when a user presses an ejection button for a device that is
currently in use or is otherwise currently incapable of being ejected.
In this case, the processing of the ACPI Eject Request notification by
OSPM fails. OSPM may indicate this failure to the platform through the
invocation of the _OST control method. As a result of the status
notification indicating ejection failure, the platform may take certain
action including reissuing the notification or perhaps turning on an
appropriate indicator light to signal the failure to the user.