The x264/x265 encoders UI is a graphical front end for the x264 and x265 encoders.
Input file → i - shows the media info of the input file.
Demuxer - use auto. For MKV use "raw".
Tune - optimizes the encoder for a specific type of content.
Frame rate - the frame rate of the source video.
Set fps - required if the source is an MKV file.
Fake interlaced - required if the source is 25/29.97 progressive.
Keyframes - auto is recommended unless you need to encode angles.
colorprim/transfer/colormatrix - use bt470bg for 720x576, smpte170m for 720x480 and bt709 for everything else. Custom means that the parameter is not added to the command line, so you can specify whatever you need in "Other parameters".
Max bitrate - the maximum bitrate. Recommended: no more than the bitrate + 35% (25% is better). Set it equal to the bitrate in case of a buffer underflow.
Bufsize - the size of the buffer. Recommended: equal to the maximum bitrate, but it cannot exceed 30000.
Frames - lets you encode only a part of the file.
Input file → i - shows the media info of the input file. If a YUV file is specified, ffmpeg is not used (direct encoding).
Output - the output HEVC or YUV file. If you specify YUV as the output, your video is converted into YUV without being encoded, using the pixel format specified below. Please note that one minute of 24 fps YUV takes 18 GB.
Frame rate - the frame rate of the source video.
Max bitrate - the maximum bitrate. Recommended: no more than the bitrate + 35% (25% is better). Set it equal to the bitrate in case of a buffer underflow.
Bufsize - the size of the buffer. Recommended: the bitrate + 40%.
Tune - optimizes the encoder for a specific type of content.
HDR x265 parameters - use smpte-st-2084 instead of smpte2084 with old x265 versions (for example with x265-64bit-10bit-2017-03-11.exe).
Pixel format - the pixel format used for YUV (default: yuv420p).
Frames - lets you encode only a part of the file. Applicable only if the input file is YUV.