Documentation of a.out?

I received this question from a VSTa inquirer and I don't have a
satisfactory answer.

From: Fellow Chiou <fellow@iii.org.tw>
To: jeske@uiuc.edu
Subject: Documents of a.out!

Hi!
    I know the execution file's format of VSTa is a.out, but I can't
find any related documents or references of a.out. The only document I
got was from the general UNIX
by typing man a.out. Unfortunately it is too rough, could you tell me
where I can find
some detail informations or books of a.out. Thanks...

Fellow Chiou

TAIWAN
Received on Thu Oct 9 07:57:36 1997