I do comprehend it. If there is a FISA authorized "wiretap" on a foreigner, and an US citizen is identified, then that recording must be quarantined and purged, unless a separate court order approves keeping the recording. .
The top secret rules that allow NSA to use US data without a warrant
The NSA is empowered to retain data for up to five years and the policy states "communications which may be retained include electronic communications acquired because of limitations on the NSA's ability to filter communications".
Even if upon examination a communication is found to be domestic ā entirely within the US ā the NSA can appeal to its director to keep what it has found if it contains "significant foreign intelligence information", "evidence of a crime", "technical data base information" (such as encrypted communications), or "information pertaining to a threat of serious harm to life or property".
Domestic communications containing none of the above must be destroyed. Communications in which one party was outside the US, but the other is a US-person, are permitted for retention under FAA rules.
The minimization procedure adds that these can be disseminated to other agencies or friendly governments if the US person is anonymised, or including the US person's identity under certain criteria.