Copy and Paste are almost never the right steps to use to copy data from one FileMaker Field to another, no matter whether the fields are part of the same record, or different records or even from different tables.
Bot steps only work if the field they refer to is physically present on the current layout and browse mode access to the field is allowed. (Change behavior settings in the Inspector to deny browse mode access and the the script steps no longer work.)
It's almost always a better option to use Set Variable in place of copy and set field in place of paste (with set field referring to the variable used in the set variable step.
Another draw back to copy is that it destroys any data that your user may have copied--perhaps while working with a completely different application, and replaces it with the data copied by the script.
If your user copies some data from a web page and this is interrupted by the need to work with your database and after working with it, they return to the first task only to find that they can no longer paste the data that they copied, it tends to confuse and irritate them.
so in my script (KML Maker) - would using Set Field wipe out all the content that was being stored in "KML Output" ?
Is there an option for Set Field to append to the content rather than overwrite it all?
Set Field [YourTable::KML Output ; YourTable::KML Output & "new data goes here" ]
Set Field [YourTable::KML Output ; List ( YourTable::KML Output ; "new data goes here" // separates existing and new data with a return ]
and other expression also are possible.