Exercises are composed by one or more blocks which can also be of different type, but usually they should be of the same type (multiple choice, true/false or short answer). The idea of having different blocks is to control the flow of the logic of the questions. If you allow everything to be permuted, then the exercise could make no sense, so with the blocks you can control which questions go first and if there are some questions that can be permuted you can fix that block.