2.0/Other DynaForm Fields
Hidden fields are used to store data in the DynaForm which is unseen by the user. Like normal DynaForm fields, hidden fields create case variables of the same name, so hidden fields can be used to pass data to or from the DynaForm which the user doesn't need to see.
getField("RunningTotal").value += extraExpense;
After the DynaForm is submitted, a later trigger could access the changed @@RunningTotal and see the new value added by the DynaForm.