What is the difference between "Allow Rating" and "Enable Overall Rating" in a performance review cycle setup

Allow Rating allows users to give individual ratings, and then an average rating is calculated based on the ratings that have been provided for each question.

Enable Overall Rating allows users to give an overall rating in the entire performance review, which can override the average rating that the system has calculated. If you select the Enable Overall Rating option, then the average rating that has been calculated will be overridden.