The PSAT 10/11 is taken by 10th and 11th graders in high school, generally in October. 11th graders use the PSAT 10/11 to compete in the National Merit Scholarship Competition. The PSAT 8/9 is for 8th and 9th grade students as a “pre-pre-SAT.” Both tests are structured exactly like the Digital SAT for college admissions but assess slightly lower-level concepts in math and English.
If you are a 10th or 11th grade student, do not use the PSAT 8/9 as it will be too easy and set unrealistic expectations. If you are an 8th or 9th grader, you can use the PSAT 10/11, but just know that it should be difficult for you given that it assesses math and English concepts, which you may have never seen before.