Guide to LaTeX (4th Edition) (Tools and Techniques

442 Appendix D. L AT E X Programming
This system of version checking allows a document to insist that
suitable versions of the class and package files are loaded. It assumes,
however, that all later versions are fully compatible with earlier ones.
There is a further identifying command for general files, those to be
loaded with \input.
\ProvidesFile{file name}[version]
There is no checking of the name or version in this case, but both pieces
of information will be printed by \listfiles.
D.2.2 Loading further classes and packages
In the main document file, classes are read in by means of the initializing
\documentclass command, and packages with \usepackage. Within
class and package files, the commands
\LoadClass[options]{class}[version]
\RequirePackage[options]{package}[version]
\LoadClassWithOptions{class}[version]
\RequirePackageWithOptions{package}[version]
must be used instead. The first allows one class file to load another, with
selected options, if desired; the second permits class and package files to
load other packages. Only one \LoadClass command may appear within
any class file; it may not be called from a package file. Neither command
may be invoked in the document file. The packages argument may be a
list of several package names, separated by commas.
The ...WithOptions variants load the class or package with all those
options that were specified for the current one, something that is often
required.
How the optional version arguments interact with the corresponding
\Provides.. command has been explained in the previous section; how
the options argument is treated is described below.
D.2.3 Processing options
Both classes and packages may take options which are defined with
\DeclareOption{option}{code}
where option is the name of the option and code is the set of instructions
that it is to execute. Internally, a command named \ds@option is created.
Often the code does nothing more than set flags, or input an option
file. (\RequirePackage may not be used within the option code!) Two
examples from article.cls are
\DeclareOption{fleqn}{\input{fleqn.clo}}
\DeclareOption{twocolumn}{\setboolean{\@twocolumn}{true}}