Full Idea
ZFU set theory talks about physical objects (the urelements), and hence is in some way about the physical world.
Clarification
'Ur-' means basic
Gist of Idea
ZFU refers to the physical world, when it talks of 'urelements'
Source
Charles Chihara (A Structural Account of Mathematics [2004], 11.5)
Book Reference
Chihara,Charles: 'A Structural Account of Mathematics' [OUP 2004], p.336
A Reaction
This sounds a bit surprising, given that the whole theory would appear to be quite unaffected if God announced that idealism is true and there are no physical objects.