Stencils are different applied designs to items. They add permanent, non-recolourable decals, logos and texture effects to the items. A lot of store items and even certain expansion pack content has missing stencils. Let's take this item for instance:
Hanging Kimono Wall Art
http://na.llnet.sims3store.cdn.ea.com/u/f/sims/sims3/sims3store/objects/sculptureWallAsianLivingKimono2x1/Thumbnail_300x300.png
When you download from the store, the only preset the item will have will be the one you see on the display image even though the item SHOULD have these other stencils:
http://img.photobucket.com/albums/v257/Havelock/Kimono.png
I hope this makes sense. :D
