This RISC OS command sets the specified file to the specified type. Type is usually &FFF for ASCII text files.
Under RISC OS, a side-effect of this command is the output file, if open, will be closed.
Specifics
Under Windows, this command will recognise &B25 (.dpd), &FAF (.html), and &FFF (.txt) and it will convert this to an extension and set this extension to the correct type.
It is much better to set the correct filename (with extension) when the file is opened.
This command is necessary under RISC OS where the filetype is metadata separate to the filename, however Windows works differently.