In our environment I did replace tl_step by a user defined function which has the same input parameters as tl_step (so a find+replace through all existing scripts was an easy task).
This user defined function does a tl_step and a file operation as Olav described (including Tony's corrections).
As we were using different test automation tools we decided to write the test results into a CSV-file (with fixed syntax) which can be imported into our test management tool. And that's what this user defined function does.
FO_MODE_APPEND is the conastant name that contains the value for the append mode - at this point is it 2.
Granted WR will more than likely never revamp and change what the file_open function uses for the append mode constant, but it is none the less good programming practices to use the given constants from a language just in case they change in any future releases. It means no code would have to be changed if the value needed to be changed.
It is just like why you don't hard code file paths...if they change, then you have to go into every script and change every call that references the file instad of using it as a variable condition where it is changed once.