Remuxing vs. Transcoding
Remuxing changes the container without touching the compressed audio/video data — instant, lossless, but only works if the codecs are already compatible with the new container. Transcoding (also called converting or re-encoding) decodes and re-compresses the actual picture — always possible, but takes real time and can affect quality.
The one-sentence version
Remuxing changes the wrapper; transcoding changes the contents. If a video plays fine in one container but not another, and the codecs are supported by the target container, a remux can fix it in seconds. If the codecs themselves need to change, transcoding is the only option — see What Is Remuxing? for the full technical explanation of why.
How to tell which one a converter actually did
Transcoding takes noticeably longer for anything beyond a few seconds of video, since it's re-compressing every frame — a remux finishes almost instantly, limited by disk I/O rather than CPU. FileBench's compatibility checker can tell you in advance whether a specific file/container pairing would remux or require transcoding.
When you don't get a choice
FileBench's Remux Video tool only remuxes — if the source codecs aren't supported by the target container, the job fails rather than silently transcoding instead. Use Compress Video (or a specific converter like WebM to MP4) when you need the operation to succeed regardless of codec, accepting that it may transcode when a remux isn't possible.
If I just need to change a file extension from .mkv to .mp4, is that remuxing or transcoding?
It depends entirely on the codecs inside the MKV, not the extension itself. If the video/audio codecs are on MP4's supported list (e.g. H.264 + AAC), it's a remux. If not (e.g. VP9 or Opus), it must be transcoded — changing the extension alone, without re-encoding, would produce a file that doesn't actually play.