The \include command is used in conjunction with the \includeonly command for selective inclusion of files. The file argument is the first name of a file, denoting file.tex. If the file is also in the file list of the \includeonly command or if there is no \includeonly command, the \include command is equivalent to
\clearpage \input{file} \clearpage
except that if the file file.tex does not exist, then a warning
message rather than an error is produced. If the file is not in the
file list, the \include command is equivalent to \clearpage.
The \include command may not appear in the preamble or in a file read by another \include command.