Format overview
What is an FLV file?
Flash Video, usually stored with the .flv extension, was built for streaming video through Adobe Flash Player. Depending on when and how it was encoded, an FLV file may contain Sorenson Spark, VP6, or H.264 video with MP3 or AAC audio.
F4V is a related ISO base media format that Adobe used for newer H.264 and AAC content. The two extensions are associated with the same Flash-era delivery ecosystem but are not identical containers.
Compatibility
FLV playback today
Flash Player reached end of life, and current browsers do not play FLV directly. Desktop players such as VLC can still open many local FLV files. Playback failures commonly come from an unsupported codec, an incomplete download, or damaged metadata.
For a website, do not restore an FLV workflow. Current browser delivery normally uses MP4 or WebM files, or segmented streaming such as HLS.
Practical use
When should you convert FLV?
Convert archived FLV media when it must play in a browser, on a phone or TV, or inside a current editor. H.264 MP4 is the broadest general replacement. WebM is useful for web delivery, while MOV may be preferable for editing.
If the source already contains H.264 and AAC, a compatible remux may avoid recompressing the tracks. Otherwise, transcode with settings that match the source frame rate and dimensions.