The actual command, including its formal definition posted by @JoshLee is contained in the LaTeX 2e source, section 61.2 Sectioning (p 283). It forms part of
ltsect.dtx, the bundle containing all sectioning commands for LaTeX. It even includes a pseudo-code interpretation of the actual macro. Here is an extract:
command is the mother of all the user level sectioning commands. The
part after the
*, including the
* is optional.
- name: e.g.,
- level: a number, denoting depth of section - e.g.,
chapter = 0,
section = 1, etc.
- indent: Indentation of heading from left margin
- beforeskip: Absolute value = skip to leave above the heading. If negative, then paragraph indent of text following heading is
- afterskip: If positive, then skip to leave below heading, else negative of skip to leave to right of run-in heading.
- style: Commands to set style. Since June 1996 release the last command in this argument may be a command such as
\fbox that takes an argument. The section heading will be supplied
as the argument to this command. So setting
#6 to, say,
\bfseries\MakeUppercase would produce bold, uppercase headings.
* is missing, then increment the counter. If it is present, then
there should be no
[<altheading>] argument. The command uses the
secnumdepth. It contains a pointer to the highest section
level that is to be numbered.