What is a sematic data model?

Stephen Green wrote:

>Is it maybe time to focus on standardizing mappings
> and transformations from XML to JSON? 

Michael Kay responded:

> Unfortunately, converting a good XML design to a 
> good JSON design can only be done with knowledge 
> of the semantic data model: mechanical transformations 
> (in either direction) done without knowledge of the 
> data model always produce a mess.

Michael, what is a "semantic data model"? Would you give an example of a semantic data model for XML and an example of a semantic data model for JSON, please? How are their semantic data models used to transform XML to JSON and vice versa?

/Roger