The usual usage is to inform the reader that any errors or apparent errors in quoted material do not arise from errors in the course of the transcription, but are intentionally reproduced, exactly as they appear in the source text. It is generally placed inside square brackets to indicate that it is not part of the quoted matter.
Sic may also be used derisively by the proofreader, to call attention to the original writer's spelling mistakes or erroneous logic, or to show general disapproval or dislike of the material.
Though occasionally misidentified as an abbreviated word, sic is a Latin adverb used in English as an adverb, and, derivatively, as a noun and a verb.
On occasion, sic has been misidentified as an