Have seem a lot of usages with "ref" attributes when setting up Sitecore Lucene Indexes. Normally it's used to refer to another section of the config file. But I wanted to do more and couldn't find any more information about it. After some guessing and decompiling of Sitecore Dlls, here are some information I got.
  • Attribute "ref" can be used to refer to another section of the configuration files in Sitecore.
  • The value of attribute "ref" is using Xpath.
  • Attribute "ref" are being used to create objects in all places and eventually come to this method:

public static object CreateObject(XmlNode configNode, string[] parameters, bool assert, IFactoryHelper helper)
{object obj = (((Factory.CreateFromFactory(configNode, parameters, assert) ?? Factory.CreateFromFactoryMethod(configNode, parameters, assert)) ?? Factory.CreateFromReference(configNode, parameters, assert)) ?? Factory.CreateFromTypeName(configNode, parameters, assert)) ?? Factory.CreateFromConnectionStringName(configNode, parameters, assert);}

  • When obj is still null, it returns the value as string.
So this is always being used to create a obj to assign to a property of the parent node.
when a node has 'hint="list:xxx"', it means it's mapping to a method of the parent node's type("type" atttibute). (please ref to this Sitecore configuration factory and dependency injection article on Sitecore Blog for more details) Hope that helped you. Now if you still have time, you can keep reading to learn my story.
First, when setting up Sitecore 7 Lucene Index, you must have come across this line in the config:

<Analyzer ref="contentSearch/indexConfigurations/defaultLuceneIndexConfiguration/analyzer" />

This works great, but I wanted to use it for "ExcludeField" fields as well to keep my custom index configuration clean. The default configuration looks like this:

<exclude hint="list:ExcludeField">
<__display_name>{B5E02AD9-D56F-4C41-A065-A133DB87BDEB}</__display_name>
</exclude>

So I have tried to do this:

<exclude ref="contentSearch/indexConfigurations/defaultLuceneIndexConfiguration/exclude[@hint='list:ExcludeField']" />
and:
<exclude hint="list:ExcludeField" ref="contentSearch/indexConfigurations/defaultLuceneIndexConfiguration/exclude[@hint='list:ExcludeField']" />

None of them worked. Though the xpath is correct, but "ExcludeField" is a method and when you use "ref", it tries to create an object as a property, so it fails.
Therefore my conclusion is that you can't use "ref" in configuration files for referencing methods. This is bad news for my clean configuration initiative, I still need to do more digging on this, will post it in another post which will focus on Sitecore Custom Lucene Indexes.