This is the relevant part of the regs:
(e) Alien’s Qualifications . Detailed descriptions of the alien's prior year of employment abroad and of the intended employment in the United States are required from the petitioner to determine if the alien was and will be employed in a managerial, executive, or specialized knowledge capacity.
To document the alien's employment abroad and the alien's intended employment in the United States, a letter signed by an authorized official of the petitioner describing the prospective employee's employment abroad for the requisite one year and the intended employment in the United States, including the dates of employment, job titles, specific job duties, number and types of employees supervised, qualifications for the job, level of authority, salary, and dates of time spent in the United States during the qualifying period. In cases where the accuracy of the statement is in question, the director may require other evidence, such as wage and earning statements or an employment letter from an authorized official of the employing company abroad.
Steve.