What is FLAC?
Lossless compression for audio: about half the size of a WAV, with every sample bit-for-bit intact. It's what people archive CDs into and what hi-res download stores sell. If a conversion starts from FLAC, it's starting from a perfect copy.
What is AIFF?
Apple's uncompressed format, the Mac counterpart to WAV, and still what Logic and older Pro Tools sessions expect. Its one genuine oddity is storing the sample rate as an 80-bit extended float, a data type almost nothing else in computing still uses.