Through the "CDEF->CNTL->ITEM" hierarchy, the items you create on your screen inherit your designated mix of properties and functionality that the parent CDEF is capable of offering.
The CNTL resource (which can be shared by any number of items in any number of windows) instantiates* the many possible properties of the CDEF just a bit. A control's relative width and height are set here. If the CDEF is text-oriented, the font and size and line height can also be set here.
By creating an item on a dialog or window, you finally start manifesting the potential of the CDEF into something with which your user can interact.
See ‚àè "CDEF Inheritance" for a visual display of this fascinating procedure.
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* Makes the abstract concrete; in this sense, it makes it possible to use a property of the CDEF.