Who's covered under FMLA's 'key employee' exception?

FMLA allows companies to deny reinstatement to certain upper-level employees. Who are they?
According to the Department of Labor, a person is a “key employee” if “substantial and grievous economic injury” will result if he or she takes leave and is reinstated. Those employees must be salaried and paid in the highest 10% of all employees in the company.
When key employees request leave, the employer must notify them of their status and explain what it means at the time they ask for leave. If the company can’t tell them at that point, the employee must be notified as soon as possible, and a reasonable amount of time must be allowed for them to try to return to work.