Are there any criteria of work necessities to skip the annual leave of employee?

Article 78 of the UAE Labour Law states:

"The employee shall receive his basic pay in addition to housing allowance, if any, for the annual leave days. However, if the exigencies of work necessitate that the employee works during his annual leave in whole or in part, and the period of leave during which he has worked has not been carried forward to the next year, the employer ought to pay him his wage in addition to cash in lieu of leave for his working days based on his basic pay."

Are there specific criteria needed for exigency to be established?

E.g. in a professional service industry, will exigency only be established by the principal directly asking the employee to do that work during annual leave?

Or will exigency be established by an official deadline regardless of explicit instructions by the principal to do the work?

In the above scenario, the employee is the only person that may attend to the work and missing the official deadline will negatively affect the end client.