Sequence Number translator


I’m a newbie of audio/video multipart application, and working on licode currently. About the SequenceNumberTranslator I have some questions as follows:
1)Why needs to do ‘sequence number translation’? What is the principle/mechanism/specification ‘sequence number translation’ is in accordance with?
2)The project contributor say “Add a new Sequence Number translator for Simulcast/SVC”, why needs to do ‘sequence number translation’ in RtpSlideShowHandler, RtpPaddingRemovalHandler, RtpPaddingGeneratorHandler?

I really hope that someone can explain to me about these questions in detail. Or, give me some material Link.
Thanks a lot!