In each of the three sections (Quant, Verbal, Data Insights), you can bookmark as many questions as you wish as you go through them. Once you have seen all questions in a section and if time remains, you will be taken to a 'Review Screen.' You can click on any question to view it again, but you can only change the answer for a maximum of 3 questions per section. This feature is intended to reduce test anxiety and allow for a quick second look at difficult problems, but it does not allow you to go back to a previous section once finished.