It’s been quite some time since I paid any attention to the application of FATCA with Swiss Banks and how information of U.S. taxpayers' accounts are reported to the IRS under FATCA. To provide a high-level summary, for U.S holders of accounts not consenting to automotive disclosure to the IRS of account information, those banks are required to make aggregate disclosures to the Federal Tax Administration ("FTA") which provides the aggregate information to the IRS. For Swiss Banks that report in the aggregate, the IRS may make “group requests” through the Administrative Assistance procedure that requires the Swiss Banks to disclose the individual account information. The Swiss Federal Tax Administration discussion of the process is here; the IRS discussion and links describing the general process is here; the IRS description is here Individual account owners are notified of the request either by notice to the notice information with respect to the account or by publication in Switzerland and may appeal (good luck with that).
Group requests are requests requiring the FTA and the Swiss Banks to get information through account characteristics where the name of the account holder is not known to the requesting authority (here the U.S. competent authority). I don't know what characteristics a provided in some format like database fields, but imagine that the fields may include (i) amount in each account on the FBAR reporting date; (ii) high amount during the report year; (iii) whether the client had some type of no mail instruction or mail instructions for a non-US address; and other similar characteristics. The IRS through the competent authority could then ask, for example, for (i) all accounts which in the aggregate for the Bank equaled or exceeded $500,000 on the reporting date or during the year; or (ii) all accounts with aggregate amounts of $250,000 for accounts any of which had a no mail instruction or mail to a non-US address. There are a number of other characteristics the IRS might specify that would "mine" the "have value" targets requiring that the Banks disclose.