FLV is a legacy video format
FLV was common for Flash video. Many modern browsers and platforms no longer treat it as a normal upload or playback format.
Choose AVI or MP4 by destination
Use AVI when an older editor or archive workflow asks for it. Use MP4 when the goal is modern playback, social uploads, browser preview, or phone compatibility.
Check old clips carefully
Legacy videos can have unusual codecs, low resolution, missing audio, or damaged sections. After conversion, check duration, audio sync, and the first and last seconds.