The term SEO can also refer to "search engine optimizers," a term adopted by an industry of consultants who carry out optimization projects on behalf of clients, and by employees of site owners who may perform SEO services in-house. Search engine optimizers often offer SEO as a stand-alone service or as a part of a larger marketing campaign. Because effective SEO can require making changes to the source code of a site, it is often very helpful when incorporated into the initial development and design of a site, leading to the use of the term "Search Engine Friendly" to describe designs, menus, content management systems and shopping carts that can be optimized easily and effectively.