The @deffn command and the other definition commands
enable you to describe functions, variables, macros, commands, user
options, special forms and other such artifacts in a uniform
format.
In the Info file, a definition causes the entity
category--`Function', `Variable', or whatever--to appear at the
beginning of the first line of the definition, followed by the
entity's name and arguments. In the printed manual, the command
causes TeX to print the entity's name and its arguments on the left
margin and print the category next to the right margin. In both
output formats, the body of the definition is indented. Also, the
name of the entity is entered into the appropriate index:
@deffn enters the name into the index of functions,
@defvr enters it into the index of variables, and so
on.
A manual need not and should not contain more than one definition for
a given name. An appendix containing a summary should use
@table rather than the definition commands.