[Tex/LaTex] Classes and packages – what’s the difference

document-classespackages

Could someone explain the difference between classes and packages? We call prosper a class and amsmath a package. I couldn't find out the difference.

Best Answer

See section 2.3 of LaTeX2e for class and package writers, "Is it a class or a package?" It states the following "rule of thumb":

If [new] commands could be used with any document class, then make them a package; and if not, then make them a class.

The following quote is somewhat more enlightening:

[A] company might have a local ownlet class for printing letters with their own headed note-paper. Such a class would build on top of the existing letter class but it cannot be used with any other document class, so we have ownlet.cls rather than ownlet.sty.

The graphics package, in contrast, provides commands for including images into a LaTeX document. Since these commands can be used with any document class, we have graphics.sty rather than graphics.cls.

`scrextend`

This package makes basic features of KOMA-Script classes available for other classes, so it extends them. It is mainly meant for document writers, but could also be used by package and class writers. Description is found in main KOMA-Script documentation.

As explicitly stated in documentation this package should not be used together with a KOMA-Script class.

Feature overview:

Early or late option selection: Options can be given in key-value pairs, and not only with \documentclass respective \usepackage, but also later in preamble and most options even in document using \KOMAoption or \KOMAoptions.
Enhancement of draft mode: Provides marks for overful lines also for classes, where this is originally not defined, cf. What does the draft mode change?
Document titles: This feature must activated as package option, but then one can use the enhanced KOMA-Script features for \maketitle and environment titlepage.

See How to span \textit{} and \Large several lines in \author{} and compare my answer with the others or float left figure in titlepage (scrextend at the end of answer).
Fontsize selection: Choose any desired fontsize instead of the limited set of standard classes, it is also more flexible than package extsizes. Of course, a scalable font must be loaded, either explicitly or by loading fontenc and having cm-super installed.

See this:
```
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[fontsize=14pt]{scrextend}
\usepackage{kantlipsum}% for some filler text
\begin{document}
\kant[160-164]
\end{document}
```
See also How to specify font size less than 10pt (or more than 12pt)? or second part of lockstep’s answer to How to set a small default font size with beamer?.
Enhanced text markup opportunities: A \textsubscript is provided, and for a limited set of elements the commands \setkomafont, \addtokomafont etc. can be used, where in standard classes often internal commands must be redefined. Supported elements are title (and disposition, that will be only active for title markup), all elements for footnotes, dictum and all elements for labeling environment.

See in How to span \textit{} and \Large several lines in \author{} (already linked above) and Description-like environment with fixed labels width.
Detection of even and odd pages: A command \ifthispageodd is provided.

Example: If Then Else for odd page/even page.
Commands for interleaf pages: Additons to \clearpage offering more flexibility.
Footnote features of KOMA-Script, but for some features the command \deffootnote must have been used for activation.

Examples with \deffootnote: Layout of multiple lines footnotes, Different formatting of footnote mark in text and in footnote, Footnote number in braces / parentheses and How to set superscript footnote mark in the text body but normalsized in the foot?. In Reference different places to the same footnote you can see the command \footref in action.
Dicta, aphorisma, slogans for chapters and parts.
Enhanced decription list labeling.

Examples: Make the lines of a description item line up, align list of symbols with dash and Description-like environment with fixed labels width (already linked above).
Simple margin changes with (trivlist) environment addmargin.

Examples: Write text with some fixed amount of space from the margin, How can I indent a block of text for a specified amount?
Margin notes like with package marginpar. Do not mix up with package marginnote also written by Markus Kohm.

Best Answer

Related Solutions

[Tex/LaTex] What’s the difference between \RequirePackage and \usepackage

[Tex/LaTex] Using KOMA-Script packages with other classes

scrextend

Feature overview:

Related Question

`scrextend`