Libsubsusf_plugin.dll file is part of LibVLC plugin product developed by VLC media player. It is responsible for VideoLAN functionality. This part is required for LibVLC plugin to function. If you are planning on replacing libsubsusf_plugin.dll with a new version make sure that you backup your old file. Replacing libsubsusf_plugin.dll file with a diffent version can cause LibVLC plugin to malfunction.