If you need the candidate to add any additional units to say their monthly timesheet, this can be achieved by using the Stand By (unit) Rate code type.
Now, on the assignment select this new rate...
The candidate will have this option as part of their daily timesheet entry however, if for example you only want them to add one unit to the whole month they will be able to leave this field Zero for standard days but enter the additional units for the last day only.
The timesheet will show the additional units:
When you come to validate the timesheet you will now see the additional costing line.