SBV is YouTube's legacy subtitle download format — `H:MM:SS.mmm,H:MM:SS.mmm` start/end timestamps separated by a comma, then caption text on the next lines. YouTube Studio still exports captions as SBV when you click 'Download captions' in the video editor. Single-digit hours, period decimal, no header.
How to open
Any text editor. Most subtitle editors (Subtitle Edit, Aegisub) auto-detect SBV from the timestamp shape. Convert to SRT/WebVTT for use outside YouTube.