FLAC files are large, and Apple Music, many car stereos and older players will not play them. AC3 is what DVD authoring tools, TVs and home-theatre systems expect for surround sound. FLAC is at home in archiving music at full quality; AC3 is the usual choice for surround-sound film audio. AC3 is lossy, so keep the FLAC original if you may need full quality later.
The defaults suit most files. Under Options you can choose the bitrate and trim the start and end.
This pair uses the compatibility engine, ffmpeg built for the browser, because AC3 cannot be written by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB. AC3 is written at 192 kbps unless you choose another bitrate.