File with .CLS extension stores in text form information about typeset configuration for LaTeX document markup language. They enable users to effortlessly import document template to TEX document. Document files are imported only once on startup as instructed with the command documentclass{…}. Templates are usually a part of LaTeX installation files, but users can add custom templates. Styles work in a similar fashion and can overwrite class properties.

CLS files store general templates for basic document types such as articles/papers/publications, forms, books or reports. They help users create documents with the same style and structure. LaTeX is widely used for creating academic papers and scientific journals. Journal publishers may provide a CLS file to submitting authors to guarantee consistency of published papers.