Performance review process can include feedback from peers (within company) and external reviewers (outside company). The option to do so can be enabled while making the plan in performance setup by the Administrator.
Menu Path : Team > Performance Management Review tab
Accessible to : Employees / Assessors / Reviewers
When Peers / External Reviewers are mapped an alert is sent to them informing them of the activity to be done and the time window available.
Refer article on Updation of Peer feedback and Updation of External Reviewer feedback for details on how feedback can be given by Peer and External Reviewer.