Should OriginalVersion attestations attribute be mandatory?

First of all, thank for your great work!

According to OpenEHR specification 
www.openehr.org/releases/1.0.2/architecture/rm/common_im.pdf
and XML Shemas
https://github.com/openEHR/reference-models/blob/master/models/openEHR/Release-1.0.2/XSD/Version.xsd
attribute attestations in OriginalVersion class is not mandatory. Is there any reason for it to be mandatory?