The LispWorks Editor uses
to read and write files. On reading a file, the external format is remembered and used when saving the file. On writing a Unicode (UCS-2) file, the Byte Order Mark is written.
It is possible to insert characters in the Editor (for example by pasting clipboard text) which are not supported by the chosen external format. This will lead to errors on attempt to save the buffer. You can handle this by setting the external format appropriately.
See the LispWorks Editor User Guide for more details.