Which subtitle formats does an IPTV player support (SRT, VTT, ASS)

The most common subtitle formats are SRT, VTT and ASS — a good IPTV player recognizes and converts all of these automatically, so you don't need to know anything about them yourself.

SRT

The most common, simple text format: timecodes plus plain text. Supported by virtually every subtitle source and player.

VTT (WebVTT)

Similar to SRT, but designed for use in browsers/web video. Functionally nearly identical to SRT for the viewer.

ASS (Advanced SubStation Alpha)

Supports advanced formatting (positioning, colors, font effects), often used in anime fansub releases. A good IPTV player automatically converts ASS to a playable format, including the basic formatting.

Which format should I choose when downloading

You usually don't need to choose yourself — when downloading via OpenSubtitles, the app automatically picks up the available format and converts it correctly. Just pick the language and release that fits best.