It's a standard gmerlin configuration dialog, which is built dynamically depending on the stream layout of the source and the encoder setup. On the left you see a tree structure of all settings:
If you select “postprocess only”, the input file will be sent directly to the postprocessing plugin. Enable this for files, which already have a format supported by the postprocessor.
Finally you can choose to transcode only a certain selection given by
the start- and/or end time.
For Audio and video streams, you can also set up filters.
For text subtitles, the render options let you configure the font and colors of the subtitles, if they are converted to overlay subtitles.
The default format of subtitles is for the rare case, that you want to convert subtitles to a video stream. It's only used if you encode subtitles but no corresponding video stream.
The last tree- node for each stream has always the codec option of the output plugin for the respective streams. Here, you can change encoder settings for each stream separately.