By default SQL Server automatically creates the internal tables for Full-text search, XML indexes and Service broker. These are also created when a user query is written poorly that uses tempdb heavily. At they are called internal tables but do not contain any user related data and schema for these tabls are always fixed. You will not be able to alter the schema at any time, also you should not or cannot refer the same within the TSQL statements you execute, such as referencing SELECT * FROM <sys.internal_table_name>...(
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Read the complete post at http://sqlserver-qa.net/blogs/tools/archive/2007/11/02/internal-tables-in-sql-server-2005-what-are-they-for.aspx