Agent name delivery is an old kind of cloaking which consists of displaying dynamically an optimized specific page to search engines bots and spiders and a different version of the webpage to human visitors. This dynamic page attribution is based on the HTTP user agent identified during the webpage request made by the browser.
Originally, search engine bots had a specific agent user and differed from the one used by internet user browsers.
Nowadays, many search engine bots are hidden behind a Mozilla kind of agent and this practice becomes much less reliable.
When detected, agent name delivery process can incur penalties from search engines.