SQL Server Reporting Services 2008 R2 introduces the RDL 2010 format with enhancements such as Maps, shared dataset references, page breaks , as well as RDL expression language additions such as Lookup, and aggregates of aggregates.

You can now download the RDL 2010 specification as XPS file.  You can still download specifications for previous RDL versions.

We also published Open Specification Documentation for several Reporting Services technologies; most importantly: