This command causes all light pen sensitive fields on the screen to be made write-protected, that is, the user can select them with a light pen, but not overwrite their contents. To switch the write-protection off again, issue the command again.
For a field to be light pen sensitive, it must be displayed intensified
AD=I) or blinking
AD=B), and the first character in the field must be one
of the light pen designator characters ("?",
">" ,"&" or a
blank); selecting a field with a light pen causes the designator character to
be changed, therefore you can make the processing of fields selected with a
light pen dependent on the values of the designator characters.