The term refers to telling customers what you're not allowed to tell customers: namely, that you've been served with a subpoena for data, with attendant gag order, sometime during a given time span. This passive method of informing-by-omission is done by an ISP telling customers when the subpoena *hasn't* been served - a maneuver now legal, albeit untested in court, and Apple's one of the first big-name tech companies to try it.
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