Tablix Group Properties Dialog Box, Variables
Select Variables on the TablixGroup Properties dialog box to add custom group variables to your report. A group variable is evaluated when the first group instance is processed and can be referenced from an expression in the scope of the group or its child groups. Because the variable is only calculated one time, each reference to the variable always uses the same value.
Add a variable to the list.
Delete the selected variable from the list.
Move the selected variable in the list.
Move the selected variable down in the list
Type a case-sensitive name that begins with a letter and has no spaces. The name can include letters, numbers, or underscores (_).
Type a value for the variable or click the Expression (fx) button to write an expression.
To include a reference to a variable from an expression, use Visual Basic collection syntax. For example, the following expression specifies a group variable named GroupTax: =Variables!GroupTax.Value.