This article explains this in details:
learn.iis.net/.../url-rewrite-module-configuration-reference
Back-references to condition patterns are identified by {C:N} where N is from 0 to 9; back-references to rule pattern are identified by {R:N} where N is from 0 to 9. Note that for both types of back-references, {R:0} and {C:0}, will contain the matched string.
For example in this pattern:
www.*.com
For the string: www.foo.com the back-references will be indexed as follows:
{C:0} - www.foo.com
{C:1} - foo
Sreenshot used in this blog post uses pattern as "*", which basically means match anything and capture whatever was matched into a back reference. In that particular case both {R:0} and {R:1} will contain the same value, so you could use either one.
Sep 10 2008 by
ruslany