The SACD utilizes DSD (aka Direct Stream Digital) which is a one bit recording format. DSF and DFF are related audio files that are delivered as downloads or extracted from SACD and read by software for playback.
DSF has the ability to hold metadata, while DFF doesn't, however, some players only read DFF files. In the PCM world, DSF and DFF would be similar to having .wav and .aiff files which are both PCM based.
Now, for those of us with ISO’s from SACDs, just a reminder that they can have up to three “sections”, CD audio, SACD stereo, and SACD multichannel. However, there are plenty of stereo only SACDs and also plenty of SACDs with no Redbook CD layer.
When containing both stereo and multichannel they typically have the same program, but some may have bonus tracks in stereo, multichannel or both. They remain separate files, which contain their unique stereo or multi-channel mixes. Choose whichever version is best for your listening environment or personal enjoyment.
When it comes to file storage and playback considerations, here is the detail: Under the SACD specification, multi-channel DFF tracks are required to be compressed using DST, a lossless compression format. The compression factor is roughly 2.5:1. When these are converted to DSF, they are uncompressed, and the resulting files are much bigger when compared to the original ISO. So, multichannel is always compressed with DST, , and the stereo may also be compressed with DST too.