If omitted, the expression will be evaluated and the sort will be performed independently in each detail scope in the SortTarget.

The scope must be equal to, or contained, inside the current scope. If the TextBox has no current scope the value must be equal to or contained within the SortTarget.

The SortExpressionScope and the SortTarget must use the same data set.

Sorting takes place within the group containing the SortExpressionScope. For instance: In a table with a state group and a city group with UserSort on each header and SortExpressionScope on the corresponding group, the state sort will sort the state groups within the table and the city sort will sort the city groups within each state group without rearranging the country state groups.

Reference