There are several topics on the ALEKS Online Tests. In a relatively short period of time (60-90 minutes), ALEKS assesses the percentage of the student's current knowledge of topics from the ALEKS PPL topics (how many of the total topics has the student already mastered) by asking a small number of questions (usually 30).
ALEKS chooses each question on the basis of a student's answers to all the previous questions. Each student, and therefore each set of assessment questions, is unique. It is impossible to predict the questions that will be asked. By the time the student has completed the assessment, ALEKS has developed a precise picture of her knowledge of the course, knowing which topics she has mastered and which topics she hasn't out of all of the topics in the course.
An overview of how ALEKS works is available.