Active Directory has supported this feature since Windows Server 2003, however I only learned about it recently from Simon Garratt. An attribute scoped query allows you to search within any multivalued attribute of an entry. This becomes really useful when you want to search for users within a group without performing multiple queries.
Say you want to find all users in the sales department for a group:
So what's going on here? I'm looking for User objects stored in "CN=Group,DC=server,DC=com" and I'm using a base search scope since I'm looking within a specific distinguished name.
On the second line I call ScopeToAttribute and I give it the name of the attribute on the group (member in this case). On a side note, ScopeToAtribute is just some syntactic sugar for calling WithControls and passing in a AsqRequestControl.
On the third line I'm just creating a standard filter. Since I'm querying for users my filter will be in that context. I can search on any User property and create any projection from a User.
I think this is pretty cool since it goes for any multivalued attribute that links to other directory entries (members, groups, employees, etc.).