Html session file error

What is a "session file" in Html session file error (“Unable to open session file for writing.”)? Is this talking about opening a specific "session file" which has already been created, in which case the message is about "the session file" rather than "a session file"? Is "session file for writing" a particular type of session file, or is this message the equivalent of "Unable to open the session file in order to write."

I'd guess it refers to PHP sessions, which be default uses file for each session. Thus no special "session for writing".

Would this error message appear when somebody tried to edit a past session file ( ie reopening a previous session)? Or is it saying that it cannot open a new session file?

I wish English-speaking developers would write either "the" or "a" instead of saving a small amount of space and making everybody else guess the meaning!

Again, this just a guess, but it just says that the file cannot be written whether it exists or not. This is usually system configuration error.

 Done - documented.