Jean-Christophe Helary writes:
The only place where I could find any kind of reference about :EXTERNAL-FORMAT was the "implementation notes" on the CLISP page.
Well, the Hyperspec page about OPEN mentions it, but the only value that you can give to it portably is :DEFAULT
http://www.lispworks.com/documentation/HyperSpec/Body/f_open.htm
All the other values are implementation dependant, hence the need to look at the specific implementation notes.