Within the SAP GUI, you can create complex selection criteria in the selection screen of the query. This can be done with the Xtract RS component as well.
Please click the Edit link to define the selection for the parameters.
The dialog below will open up.
The column Sign defines whether the restriction is included in or excluded from the result set. The column Operator defines an operator (e.q. =,<,> ….). The values must be entered in the LowValue and HighValue cells. HighValue only has to be filled in if the operator is  (between) or ][ (not between). In all other cases it is sufficient if only the LowValue cell is filled in.
Of course, you can also use variables. Just place an @ symbol right before the variable name.