Thanks for the clarification, I understand now. Messages like that description are language strings and Moodle welcomes improvements to these via AMOS. I've submitted a change for this, if the change is accepted then in future updates the description will be:
Some internal Moodle variables may be automatically appended to the URL. Type the name of the URL's parameter in the text box and select the corresponding Moodle variable.
As Moodle's developers come from all over the world their first language may not be that of Moodle's default language or whichever language pack one is using. Sometimes wording may be unclear as a result.