filename.mp4.part what that mean

[TaktakTaktouk]

I'm using youtube-dl on my website to help users to download videos. The file must be downloaded in mp4 format. In my directory I find many of these files:
filename.mp4.part
Does that mean that there was an issue during the download?
What are the causes that prevent youtube-dl from downloading a video correctly?
Here is my command if that can help:
youtube-dl -o 'filename.%(ext)s' -f 'bestvideo[ext=mp4][width<?1024][height<?1024]+bestaudio[ext=m4a]/best[ext=mp4][width<?1024][height<?1024]/bestvideo[ext=webm]/best' URL

[dstftw]

.part is partially downloaded file. The reason for that may be anything.