Cudcuds said in forum #415824:
Plenty of these you could have definitely provided a source.
Like for the P&S video by Ploxy, you could have used the link from Mr Ploxy's Newgrounds page or the Pixiv page that links the preview. It certainly would be better source than what's there currently.
For future uploads, look on the artist's pages for the sources. Twitter, Pixiv, Newgrounds, etc.
The file at the source link has to match the file on Danbooru. A file from a third-party source may be different from the one at the original source, so if the Newgrounds video was downloaded from somewhere else, the Newgrounds source won't necessarily be correct. You would need to compare the video on Danbooru with the original Newgrounds video before changing the source to Newgrounds.
AnonStatus said in forum #415588:
the only other reason i can see is source but nowhere in the help:upload rules does it state a source is outright required
That's because help:upload rules is about what you're allowed and not allowed to post, not about how you should tag and source your posts. For basic tagging/sourcing expectations, you should refer to help:upload.