Initial situation
You have two approved style sets that are assigned to three mailings:
Change style set
You would now like to change your standard style set that is already being used in three mailings. A new style set draft which is linked to your approved style set is created when you click 
"Edit" in the style set.
However, the approved version of the style set is still assigned to the mailings. The changes made in the style set draft are not shown in the mailings:
Apply changes to style set to mailings
When you apply the changes in your style set to your mailings, they will also be displayed in your linked mailings.
Your style set draft is merged with the approved style set. Only the approved style set is now shown in the style set overview:
