\subsubsection, \paragraph, \subparagraph ¶Synopsis, one of:
\subsubsection{title}
\subsubsection*{title}
\subsubsection[toc-title]{title}
or one of:
\paragraph{title}
\paragraph*{title}
\paragraph[toc-title]{title}
or one of:
\subparagraph{title}
\subparagraph*{title}
\subparagraph[toc-title]{title}
Start a subsubsection, paragraph, or subparagraph. The standard
LaTeX classes article, book, and report all have
these commands, although they are not commonly used.
This produces a subsubsection.
\subsubsection{Piston ring compressors: structural performance}
Provide exterior/interior wall cladding assemblies
capable of withstanding the effects of load and stresses from
consumer-grade gasoline engine piston rings.
The default output of each of the three does not change over the
standard LaTeX classes article, book, and
report. For \subsubsection the title is alone on
its line, in boldface and normal size type. For \paragraph the
title is inline with the text, not indented, in boldface and
normal size type. For \subparagraph the title is inline
with the text, with a paragraph indent, in boldface and normal size type
(Because an article has no chapters its subsubsections are
numbered and so it looks like ‘1.2.3 title’, for
section 1, subsection 2, and subsubsection 3. The other
two divisions are not numbered.)
The * form shows title. But it does not increment the
associated counter and produces no table of contents entry (and does not
show the number for \subsubsection).
The optional argument toc-title will appear as the division title in the table of contents (see Table of contents, list of figures, list of tables). If it is not present then title will be there.
For determining which sectional units are numbered and which appear in the table of contents, the level number of a subsubsection is 3, of a paragraph is 4, and of a subparagraph is 5 (see Sectioning/secnumdepth and see Sectioning/tocdepth).
The paragraph that follows the subsubsection title is not indented, as is a
standard typographical practice. One way to get an indent is to use the
package indentfirst.
There are a number of ways to change the behavior of the these commands.
One is the \@startsection command (see \@startsection: Typesetting sectional unit headings).
There are also many packages on CTAN that address this, including
titlesec. See the documentation on CTAN.