Stars for Decoration?

[Overheal]

Can someone explain the mechanic behind decorations? They all have a star cost but no currency for this is shown.

[Semirotta]

Some you can't place even though you should, they won't always increase decoration % at all for quest

[Stapper]

The star is a rating to show how much they influence the decoration compared to each other. When you hover over one in the menu, the cost of the decoration is displayed at the top of the screen, underneath your money and crowns.