I believe you can flag most questions to go back and review. The exception are the vignettes where questions build upon one another. Once you complete a section (50 questions), you cannot return to that section.
I’m not sure if there is any more recent information that is distributed to individuals taking the exam this year – this is the most recent information I have heard and found on the website (https://specialization.apta.org/become-a-specialist/exam-specialty-resources/exam-day).