jenkins/war/resources/help/parameter/string.html

<div>
    Defines a simple text parameter, where users can enter a string value,
    which you can use during a build, either as an environment variable, or
    through variable substitution in some other parts of the configuration.

    <p>
    Note that when this field is exposed as an environment variable, it
    will be always in the upper case (e.g., the parameter "foo" would be exposed
    as "FOO"). In Ant for example you access it by typing in ${env.FOO} 	
</div>