Question about use of "sic" in "On Writing"

The Latin adverb sic added immediately after a quoted word or phrase (or a longer piece of text), indicates that the quoted words have been transcribed exactly as spelled or presented in the original source, complete with any erroneous spelling or other presentation. The usual purpose is to inform the reader that any errors or apparent errors in the transcribed material do not arise from transcription errors, and the errors have been repeated intentionally, i.e. that they are reproduced exactly as set down by the original writer or printer. Sic is generally placed inside square brackets, or in parentheses (round brackets), and traditionally in italic, as is customary when printing a foreign word. Sic may also be used as a form of ridicule or as a humorous comment, drawing attention to the original writer's spelling-mistakes and emphasizing his or her erroneous logic.

Forgive me Ed but you might not have addressed Ando's query. What he’s really asking is probably, what’s wrong or funny about the preceding word or phrase

Thank you for your reply. I don't know why the word "which" falls into that category where it need "sic" after it. One thought I had was because the word "which" was used as opposed to "that" i.e. non essential as opposed to essential, but I don't know if that makes sense.