Citators are used to determine the subsequent history of a case, statute, or regulation, and thus assess its validity. While it is perfectly acceptable to use a general citator such as Shepard's (LexisNexis) or KeyCite (Westlaw), the specialized tax research databases have their own citator systems, which can be useful:
The citator can be browsed from the above link found via the Table of Contents. The citator can be searched as well by selecting it under Editorial Materials in the Practice Areas selection. Upon viewing a source, there is also an option at the top of the page, as well as on the left menu, to view the Citator results for the source.