Flex graphic assets: SWF or SWC?

Question

Which is a better format to store graphic assets for a Flex application, SWF or SWC?

Are there any real differences, and if so what are they?

Answer 1

I have no real reason for doing this so it may be incorrect but I usually create SWF's for things that need to be loaded during runtime and SWC's for things that need to be available for design time.