PPSA28416 calls it on its existing save at boot (after sceSaveDataDirNameSearch finds it) and aborted on the missing implementation. The call takes one parameter block: user id at +0, the directory name pointer at +8. Host saves have no size limit, so the conversion completes at once: an existing save returns OK, a missing one NOT_FOUND, an invalid name PARAMETER. The title polls sceSaveDataGetEventResult once afterwards and continues on 'no event'.
Chad