Commonly supported properties

Hi all,

I'm interested in getting people's thoughts on a particular line from the
"How to publish Linked Data on the Web" tutorial. Specifically the
following...

"It is common practice to mix terms from different vocabularies. We
especially recommend the use of
rdfs:label<http://www.w3.org/TR/rdf-schema/#ch_label>and
foaf:depiction <http://xmlns.com/foaf/0.1/#term_depiction> properties
whenever possible as these terms are well-supported by client applications."

I'm curious about a couple of things in regards to this: a) does anyone know
what "client applications" the authors might be referring to, and b) are
there examples of other properties that enjoy similar support?

Thanks,
--jay