FLAC files are large, and Apple Music, many car stereos and older players will not play them. M4R is the only format an iPhone accepts as a custom ringtone. FLAC is at home in archiving music at full quality; M4R is the usual choice for custom iPhone ringtones and alert tones. M4R 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.
The conversion uses your browser's own video and audio codecs (WebCodecs), with hardware acceleration where the device has it; the file is read from disk as it goes and can be up to 2 GB. Where the browser lacks a codec, the page falls back to the compatibility engine. An iPhone ringtone is limited to 40 seconds, so only 40 seconds are kept, from the start point you set under Options (the beginning by default). M4R is written at 192 kbps unless you choose another bitrate.