It's service

An O-1 Advisory or Consultation Letter is a written opinion provided by an appropriate U.S. peer group, labor organization, or individual(s) with expertise in the beneficiary’s field. Depending on the O-1 classification and field, the consultation is generally a required component of the petition.
 
Carnegie Evaluations assists O-1 petitioners by facilitating independent advisory opinions from qualified faculty experts with relevant expertise in the beneficiary’s field. After reviewing the beneficiary’s credentials and the details of the proposed work, the expert provides an objective assessment of the nature and significance of the proposed duties, whether the work requires the services of an individual with extraordinary ability, and how the beneficiary’s accomplishments and professional standing align with the position’s requirements. Each letter is tailored to the specific facts and documentation of the case to provide a well-reasoned expert perspective in support of the O-1 petition.