An ELT Glossary : Placement tests
A placement test is a test used to determine which course a learner should attend. It would be used in eg a private language school to assign newly-arrived learners to a class at an appropriate level.
A placement test will therefore need to cover a range of items at all levels from beginners (CEFR A1) to advanced (CEFR C2). If it contained 60 items therefore, ten might be at level A1, ten at A2, ten at B1, and so on. A learner who scored highly on the first 30 questions, but whose scores dropped off immediately afterwards would therefore be seen to have assimilated the language commonly taught at A1-B1 level, and to need to study at B2 level. S/he would be placed in the class accordingly.