Each survey contains unique questions. These questions and the data returned by them, are distinguished by a unique custom variables (sometimes called a UDE). You can see this UDE if you open up any Survey and click on the question title. Sometimes the question is a pre-defined system question (where the UDE would be "email" or "identifier" etc) but for user-defined questions the UDE is usually in the form: "survey7896022".
The purpose of the UDEs is to maintain clear distinction between data items and ensure that each question (and it"s response) is handled in isolation.
The survey setup pages of the Admin Console contain a Use Existing Question button.
This function is designed to allow different questions (even on different surveys) to share the same UDE name. This means that different questions (tailored maybe to match different skills) that feed into the same UDE can be reported on together.
Many LivePerson Enterprise Enterprise clients use this function for cross-skill (or over-arching) monitoring, where the same question on each particular skill or survey constitutes part of centralized, rather than skill-based reporting.
To learn more about creating and passing custom variables click here.