WS-Fragment.Put has the following:

[Body]/wst:Put/wsf:Fragment/wsf:Value
    This OPTIONAL element contains the fragment resource representation corresponding to the subset of the resource identified by the wsf:Expression element. 

It is not clear to what should happen to any optional/missing bits of XML
not included within this Value.  For example, are any snippets of XML
currently present left unchanged?  Removed? Defaulted to some resource-specific
value?

We should follow the pattern established in Transfer, where it says:
Any optional values (elements or attributes) not specified in the Put request message will be set to a resource-specific default value. 

Proposal:
Add the following sentence to the end of the paragraph referenced above:

When @Mode is 'Insert' or 'Replace', any optional values (element or attributes)
within this subset that are not specified in the wsf:Value element will be set
to a resource-specific default value.