Adjustments will automatically be set to 'Apply Immediately' this would mean that if you was to end this staff members employment the adjustment amount would not be included in the pro-rata calculation, but would be given to the staff member on top of any pro-rata allowance (this is most likely to apply to carry over days).
If you was to untick this box, and terminated the employee then the adjustment would be included in the pro-rata calculation as part of the holiday allowance.
If you add an adjustment of 5 days and left the box ticked, then terminated the employee, Staff Squared would work out a pro-rata entitlement (either owed to the company or employee) then on top of this they would add the 5 day adjustment. So, if their pro-rata termination amount said that the company owed the employee 2 days, they would actually owe them 7 days inclusive of the 5 day adjustment.
If you was to untick this box and then terminate the employee then the extra 5 days would be pro-rated along with the rest of the employees holiday entitlement.