In brief
Peeling of the hard outer part is a separate task from the processing of the leaves.
Useful to know
When choosing an alabaster, it looks for a hard head that feels dense for its size. Smaller specimens may have a more gentle texture. Iowa State considers separately the preparation of the head and leaves, including the removal of the hard outer part before slicing. [1]
For a raw supplement, cut the prepared portion according to use: thin strips for a salad or larger pieces for another dish. These are suggestions for a form, not individual varieties of kohlrabi.
If the product is leafy, do not judge of its entire condition by them alone. Appreciate the chapter itself, and then decide what you're going to use. Individual parts do not need to go into one recipe or be processed equally long. In an unknown vegetable, the first useful question is which part you hold and what texture you want to get.
Sources
For further detail and to check the information: