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In general, any explanatory word or phrase that helps explain something that came before it.

In formal legal writing, a statement that may be added in parentheses at the end of a source citation.  Such parentheticals often explain the relevance of the source, indicate something about its persuasive force, or quote language from the source.  Many other ways to use a parenthetical exist; in general, any use of a parenthetical that could be useful for a reader is acceptable.

See Bluebook.