FLAC files are large, and Apple Music, many car stereos and older players will not play them. MP2 is the audio format broadcast and DVD-authoring tools expect. FLAC is at home in archiving music at full quality; MP2 is the usual choice for broadcast radio and television audio. MP2 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 MP2 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. MP2 is written at 192 kbps unless you choose another bitrate.