Labor burden costs are calculated using the Labor Burden % fields at the top of both your field labor and your overhead budget(s). You can have a different labor burden for overhead staff, since worker's compensation rates may be lower.
By doing it this way, LMN will automatically calculate the forecast costs of your field labor burden by adding that percentage to the wages you enter. This makes it far easier to budget for payroll costs. Otherwise, everytime you changed the # of payroll hours or a wage, or added an employee, you'd have to go back to overhead to recalculate your forecast labor burden expense totals.
You can see how much labor burden is being accounted for your field staff in the tiles at the top of the field labor budget.
The overhead budget also adds the labor burden costs using the labor burden % at the top of the overhead budget.