Apple's own transparency report, under FISA orders. Presumably it includes all subscriber data they can access for the specified accounts, so likely contacts, photos, and device backups (full iMessage chat history, or sync keys to decrypt same).

FISA orders are not warrants and do not require probable cause; the FISA Amendments Act Section 702 spying that goes on (aka PRISM internally to the IC) pulls data directly from cloud provider systems without a search warrant and was cited by Ed Snowden as one of the main reasons he came forward.