In the when Condition tab there are very limited options for selecting the fields and other When conditions in 8.4 version. Advanced tab also can be used for specifying the when conditions. Here we can use parameters and any fields that are created in the Pega. And also we can specify the conditions using some complex expressions as well by leveraging expression builder. But to use any parameters in this tab first corresponding Parameter should be declared in the parameters tab and then in this Advanced tab that parameter can be used as part of the expression. Please find the attached screen shot for your reference.