A Citator is a tool which allows you to track the history of your case and the treatment of your case by subsequent courts.  Citators allow you to determine if your case is still good law and it acts as a research tool allowing you find other cases (and other secondary materials) which cited your case.

When you shepardized/keycited U.S. v. Grabowski.  U.S. v. Grabowski is the cited reference.  All the cases that have cited U.S. v. Grabowski are the citing references.  In the lingo, your case is the “cited reference” and the cases which mention it are the “citing references.”  Grabowski = the ‘cited reference.’  Other materials which cite ‘Grabowski’ are the citing references.  

Citators are used for a number of reasons.  One reason is to find a case or statute's direct history. With cases, one can find prior and subsequent history.  With statutes, one can find whether the statute was amended or if there is pending legislation, for example.  Another reason to use a citator is to identify negative treatment (such as if the case has been overruled) or to find positive treatment (such as another case that agrees with the analysis of your case).  Citators can also be used to find additional primary and secondary sources.  Finally, there are practical and ethical considerations.  Lawyers must be thorough and prepared, and part of that involves making sure that the cases or statutes cited are still good law and can be used as precedent.

The citators in WestlawNext and LexisNexis Advanced are both: