Variable
This drop down contains a list of Configuration Variables. If you want to create a Configuration variable, see Configuration Variables.
Operation
You can choose to replace a variable's value or append/increment it. If you choose to append/increment then it takes in the value type of the variable. For text/expression type variables the new value will be appended to the old value. For numeric variable values the old value will be incremented by the new value.
New Value
Input the new value for the variable.
Expand variable references in new value
If you select this option then the value will be expanded (if there's any expressions in it) when the variable is used and the new value will be set to it.