A test or quiz is referred to as being based on "self-assessment" because the score or outcome of the instrument depends entirely on your perception of yourself, your likes, dislikes, priorities, and so on. Many of the more popular tests have been in use for over 50 years and have a substantial amount of research to verify their relative accuracy for the purposes of predicting career satisfaction.
Having said that, caution should be exercised in interpreting the results of any self-assessment instrument. Never rely fully on the results of any single tool. Using a number of test instruments reduces the possibility of distorted findings, due to any number of factors. More important than the specific career suggestions, you should look for recurring themes and patterns in the types of occupations identified. For example, a number of tests may consistently identify that you have strong analytical skills, enjoy working with people, or that you prefer to have flexibility and autonomy in your work.
In order for the results of the assessment to be as meaningful as possible, try to take the assessments when you are unhurried and in a relaxed state of mind.
And most importantly, remember that self-reflection is an important part of the process. You often know more about yourself than you realize.