Your explanation with example screen shot really helped thank you. I have a question here if i give 3 radio buttons for example and its not required field , then user can leave empty na why this "Not applicable " holding string to display.
Yes it can be left empty and it that case it will take the NULL value for the property/field.
In case you dont want it to be left NULL, but rather "0" or a string like "not applicable" (if you leave it NULL, it may not read that property for the WO when you do a obj-open for that work object. Also critical during page-copy etc).