The Reviewer is usually someone identified by the department to review requests for completeness and accuracy. This can also be the Traveler if so inclined or there is no business staff identified to fill the Reviewer role
The role is separated into PA-Reviewer for Pre-Approvals, and RB-Reviewer for Reimbursements. The same person can occupy the reviewer role for Pre-Approvals and Reimbursements.
It is up to the discretion of the department to determine how the reviewer roles will be structured to best support travel requests.