From LimeSurvey Manual
On this tab you can set which additional URL parameters will be accepted by your survey (these parameters would be attached to the general survey start URL). This is important if you integrate with a professional survey panel provider that provides the participants for your survey. They usually hand over a unqiue ID with every participant and you will need to save/handle this unique ID for reference. Also with some providers the unique ID of the participant (bdropping out due to quota or finishing the survey) needs to be sent back to the panel provider in the survey end URL.
If you like you can also save the value as a response of a "Short text question" or "Multiple short text question" type (you will be able to select any short text question to save it to). Even if you do NOT save the value of the URL parameter you will be able to use this parameter in the end URL. Please have a look at the documentation of the End-URL for further information.