In the SQL Server 2005 thesaurus XML configuration file, what is the expansion set?

Expansion sets are synonyms for the search term and returned as results if they appear along with the search term.
The expansion set is the group of values that are synonyms and can be substituted for the search term. For example, an expansion set can be "SS2K5", "SQL Server 2005", "SQL2K5". In this case, fields with any of these 3 values would be returned as a result for searches on "SQL Server 2005".