If the file exists, it is opened and the file pointer set to the end of the file so your output is appended to the end of the file. If the file does not exist, it is created.
This has the same behaviour as BASIC's OPENUP command (OS_Find &Cx); or in C, the "rw+" options to fopen().
If you want to create a new file, overwriting previous contents, refer to filewrite().
If a file is already open, an error will be generated and script processing terminated.