What is the best way in which to understand bracketed text?

Question

Though a native English-speaker, I've always had trouble understanding bracketed text. It's fairly clear that parenthesis indicate "optional" text, as in below:

The (unusually quick) man stormed into the store and grabbed every lighter from the stand near the cash register.

Above, it's not necessary to include the text "unusually quick," as it can be determined somewhat from the context, but its inclusion indicates a reaction of the author or a point desired to be emphasized. The general idea seems to be that a sentence with parenthesized text should be able to be read without the parenthesized text. While the parethesized text adds to the sentence details, it is not necessary in and of itself.

However, square brackets seem to indicate complete literary freedom to rewrite a quotation entirely:

...[he] had [indicated that he had] made the gesture without [complete knowledge and] familiarity with [Taiwanese] custom.

What are the rules with using square brackets and their general implication? I, as the reader, am completely at the mercy of the author in trusting him that he has conveyed the quote in its proper context and intention.

Answer 1

From Words Into Type, Third Edition (1974):

Brackets are used to enclose comments, explanations, queries, corrections of error, or directions inserted in a quotation by some person other than the original writer. The matter enclosed may be wholly independent of the text, or it may be words supplied to secure complete and understandable sentences, ...

From The Chicago Manual of Style, Fifteenth Edition (2003):

6.104 Use of square brackets. Square brackets (in the United States usually just called brackets) are used mainly to enclose material—usually added by someone other than the original writer—that does not belong to the surrounding text. In quoted matter, reprints, anthologies, and other nonoriginal material, square brackets enclose editorial interpolations, explanations, translations of foreign terms, or corrections. Sometimes the bracketed material replaces rather than amplifies the original word or words. ...

and:

11.14 Tenses and pronouns. In quoting verbatim, writers need to integrate tenses and pronouns into the new context. [Examples omitted.] Occasional adjustments to the original may be bracketed. This device should be used sparingly, however. [Example omitted.]

and:

11.68 Use of brackets. Insertions may be made in quoted material to clarify an ambiguity, to provide a missing word or letters, or, in a translation, to give the original word or phrase where the English fails to convey the exact sense. Such interpolations, which should be kept to a minimum lest they irritate or distract readers, are enclosed in brackets (not parentheses). ...

I can see why you might feel suspicious about the intrusion of the annotator into a pristine quotation, but perhaps it bears observing that you're at the mercy of the integrity and accuracy (or lack thereof) of the reporter, editor, and publisher, whether the quotation they supply has square brackets and ellipses marked in it or not.