We can make a DFF segment read only by restricting the segment with a valuset of ‘Special’ type.
Steps to create ‘Special’ type of value set:
Create a value set with name “XX_READ_ONLY_VS” and Validation Type as “Special”. Click on “Edit Information’ button and enter the below block of code in under ‘Edit’ and ‘Validate’ event
FND SQL "BEGIN NULL; END;"
Configure the DFF Segment with this value set to make then DFF segment as read only from application.
Here in this example I have configured two segments of “Additional Person Details” DFF with “XX_READ_ONLY_VS” value set and you can see in the below screen shot that the segments are now read only.