To have the "Display" parameter make more sense to contributors, I will be updating this to visibly show the YouTube and NicoNico video thumbnail. This will also help (youtube wise) in determining when a video is no longer available for viewing. This should be implemented during the week.
nnd_id = NicoNico (must have either "sm" or "nm" before the number to function.
yt_id = YouTube (just the number)
When neither parameter is used then there is no thumbnail. The "Displayed" parameter can still be used for those who want to link other places besides NicoNico and YouTube, or simply don't want a thumbnail to be the display.
Using two thumbnails causes the infobox to become wider than the content space. I am working to correct this but I hope it isn't too much of a problem as is. And depending on the creation of various Accessory or Stage pages, they will also have this option.