Per policy, employees should not apply for more than their annual entitlement. However, the system does not prevent it. With the allowance of going negative 40 hours, they could enter their full-year entitlement plus another 40 hours.
Based on your question, it sounds as if you are using Employee Self-Service (ESS) or a similar method for data entry. Although I'm not that familiar with ESS, there may be an opportunity to add a limit at data entry.
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