It can be compared with an objective test, which has right or wrong answers and so can be marked objectively. Subjective tests are more challenging and expensive to prepare, administer and evaluate correctly, but they can be more valid.
Example
Tests of writing ability are often subjective because they require an examiner to give an opinion on the level of the writing.
In the classroom
Learners preparing for a subjective writing test, for example a letter of complaint, need to think about their target audience, since they are being asked to produce a whole text. Teachers can help them by emphasizing the importance of analysing the question and identifying the key points of content, register, and format.
See also:
https://www.teachingenglish.org.uk/article/achievement-test
https://www.teachingenglish.org.uk/article/criterion-referenced-test
https://www.teachingenglish.org.uk/article/diagnostic-test
https://www.teachingenglish.org.uk/article/norm-referenced-test
https://www.teachingenglish.org.uk/article/objective-test
https://www.teachingenglish.org.uk/article/proficiency-test
Further links:
https://www.teachingenglish.org.uk/article/assessment
https://www.teachingenglish.org.uk/article/testing-assessment
https://www.teachingenglish.org.uk/article/test-writing
https://www.teachingenglish.org.uk/article/test-question-types
https://www.teachingenglish.org.uk/blogs/vicky-saumell/vicky-saumell-using-rubrics-assess-projects
https://www.teachingenglish.org.uk/article/evaluating-speaking-ielts-speaking-test