Smil allowed formats

Hello,
My name is Laure and I am working on the SMIL translations to other formats..
I have been looking for what kind of media formats can be included into SMIL
but I cannot find anything clear. I know that you can include video, audio,
text... but, can you use any format? Yes, I know that is the player who
has the restrictions (if it has not got the codecs, it will not be able to
reproduce it), but aren't there restrictions or rules within SMIL refering
to this? I mean, are there some formats that all the SMIL players should
cover to be SMIL Players?
Regards,
Laure.