https://www.weasyl.com/help/collections
It's a "secondary" gallery for artwork that features your character/art for you/your comissions/etc., art by other people.
It's one of the features on Weasyl to keep the site clean of double uploads/multiple uploads of the same image, which at the same time, will keep running costs lower, due to the smaller server space required to keep the site running.
Link
DonCoon
If you have someone elses work on Weasyl, then why not use the collections? Or hasn't the artist uploaded this image to Weasyl?