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".)
</div>