When we are on the assignment screen which is before decision shape, we can open the clipboard and edit, add the property and its value. So when we click on submit for this assignment the as the clipboard page has the value, it evaluates the decision shape and so that we can see how the flow is processing.
Please find the below screenshot for adding a property to the clipboard.
Declare expressions are mainly to calculate the values based on expressions. And the value of target property is calculated based on change tracking. It is generally not used for testing purposes. In the above scenario too, we are not calculating standardized scores, user will enter the score, so it is not needed to use Declare expression here.