Default value for Checkbox: I'm implementing a simplified version of IW21 where I put my input fields on the initial screen of IW21. This works fine but I have a problem with the initialization of my checkboxes. I use the option default="X" for some checkboxes. When the user clears such a checkbox and then calls the transaction again with /nIW21, the checkbox is still cleared. I would expect the default value "X" to be set again but this is not the case.
The default=... option of the checkbox command is not executed if the screen id doesn't change, which is the case if the user enters /nIW21. I suggest you remove the default= option and set the defaults with your own coding, based on the system variable V[_transactionid]. The  V[_transactionid] variable changes with each /nIW21:

if not V[iw21_transactionid=&V[_transactionid]]
  Set V[[iw21_transactionid] "&V[_transactionid]"
  Set V[checkbox1] "X"Set V[checkbox2] ""

Checkbox ... name="checkbox1"
Checkbox ... name="checkbox2"