RDF people, please define "surface syntax" and "concrete syntax"

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

I see usage of the terms "surface syntax" and "concrete syntax" in multiple RDF articles, e.g.,

    The RDF graph syntax is in several important respects simpler than any surface syntax, 
    and makes possible a very simple and straightforward - almost elementary - approach 
    to some surface-syntactic issues which are notoriously troublesome to get exactly right, 
    especially the issue of bound name scopes. [1]

and

    In this section we present an RDF concrete syntax for the rules. It is straightforward to 
    provide such an RDF concrete syntax for rules, but the presence of variables in rules 
    goes beyond the RDF Semantics. [2]

Would someone from the RDF community please define "surface syntax" and "concrete syntax"?

/Roger

[1] http://www.ihmc.us/users/phayes/RDFGraphSyntax.html

[2] http://www.daml.org/2004/04/swrl/rdfsyntax.html