@Feyona All outfit tags are used for SimInfo generators. These generators are templates for new Sims that can be placed in the world. In addition, they can be used to just create new outfits.
For example, a tag for a Tragic Clown can be applied on an outfit piece and when used in the generator, it will make an outfit that is made out of clothing tagged with that tag. And then an outfit like that is applied on a Sim that supposed to be a Tragic Clown.
For the wedding dress, the Situation tag, does nothing because outfits aren't randomly generated for the "Situation" category. If you would generate random outfits for that category, one of the possible outfits would be the wedding dress.
To make an outfit that is a wedding dress, different tags are used. The existing tag is just informational so it is known that this outfit is supposed to be used on the "Situation" outfit category.
I'm just trying to help you understand.