I'm implementing the iOS9 Search index using NSUserActivities.
According to the documentation NSUserActivity there should be a property called contentAttributeSet that is used to add more content to the search item. However looking at the NSUserActivity class in Xcode 7 shows no property with this name. I'm using Xcode 7 beta 6.
Got it now.
The contentAttributeSet is of type CSSearchableItemAttributeSet, which is a class in the Core Spotlight framework. So I had to add Core Spotlight to my project and import it in the file I was using NSUserActivity in.
Sneaky sneaky, Apple!