The documentation on Core Data entities says:

You might implement a custom class, for example, to provide custom accessor or validation methods, to use non-standard attributes, to specify dependent keys, to calculate derived values, or to implement any other custom logic.

I stumbled over the non-standard attributes claim. It's just a guess: If my attribute is anything other than NSString, NSNumber or NSDate I will want to have a non-standard Attribute with special setter and getter methods? So, for example, if I wanted to store an image, this would be a non-standard Attribute with type NSData and a special method, say -(void)setImageWithFileURL:(NSURL*)url which then pulls the image data from the file, puts in in an NSData and assigns it to core data?

Or did I get that wrong?