CGZ files

Hi All,
 
I find the draft underspecified about compressed CGM files. More
specifically, we would like to know what kind of CGM files may be
compressed?
 
Version 1 to 4?
Can I compress a WebCGM 1.0 CGM file for example?
 
Is this a WebCGM 2.1 conformance feature for viewer and authoring tools?
Or is this a new WebCGM 2.1 (and only 2.1) 'encoding scheme' ... for
lack of a better word?
 
Thanks.
Benoit.