Universal Media Server | Forum

Don't know but definitely it works for many renderers but support is almost not documented or also it differs between containers on same renderer.
Also "stupid" Panasonic TV without MKV support supports external subs as Valib found out, but only for DIVX/XVID files.
Sometimes people with e.g. Samsung TV are angry why subs are rendered by UMS but the reason is that UMS sends subs as is and renderer shows them on TV so they are surprised they must disable them on TV remote and not in UMS

In the next version (6.5.2) it will be possible to disable transcoding completely by setting this in UMS.conf: Disabling of transcoding per extension has also been fixed.