When you Read from file, you are reading the file into memory and you would reference that data with a variable like FileAccess8200_outputData. You must specify the file path and name to be read (variables are permitted).
When you Write to file, you are writing data to a file on the server and must specify the file path and name to write to (variables are permitted). When writing, you must specify the Input data that contains the information you are writing. You can enter the data in this field or reference a variable which contains your data.
When you Append to file, you are adding data to an existing file on the server. This mode is just like using Write to file except you are not overwriting the existing file.