There are fixed and variable conditions. You must select the condition type in the condition from a list.
Fixed condition
With a fixed condition, attribute values of selected objects are compared with a predefined value. The Value column can be edited.
Variable condition
With a variable condition, attributes, connections, or properties of a predefined object are compared with the attributes, connections, or properties of an object that the user selects in a dialog. As soon as you set the condition type to Variable, you can select a value in the Value column.