This is the number that appears under the uploaded images on the Media tab. It is a uniquely assigned number to every image uploaded by a user here, and can be used in the [[image:]] tag to display an image in the Wikibase.
Video System Identification Number
Works the same as the ISIN, but with videos. Videos can only be uploaded by an Admin, and can be linked using the [[video:]] tag.
Documentation for the image: and video: links can be found here.