What does ETS mean by a Computer Based Test (CBT) that is Section or Level adaptive?

Most standardized entrance exams fall into categories such as CAT (computer adaptive test), CBT( computer based test ), iBT (internet based test) PBT (pencil and paper based test). CAT means the questions in the tests are picked up based on the test takers response and the difficulty level of the question. We can say that in away in CAT the computer adapts to the skill levels of the test taker. CBT simply means that the test is conducted on computer. IBT means test is conducted using internet. In a pencil and paper based test, pencils and papers are used. GMAT is a CAT whereas GRE is a CBT. TOEFL is an iBT whereas IELTS is a PBT. Though GRE is considered a CBT, that is not fully accurate. We can say that GRE revised general test a hybrid of CAT and CBT. To be precise we can call GRE a computer based test that is section or level adaptive, means a section is chosen the candidates performance on the previous section.