Brackets are used to indicate editorial interpolation within a quotation and to enclose corrections, explanations, or comments in material that is quoted (see 8.6.1, Quotations; 8.8.6, Ellipses, Change in Capitalization; and 8.8.7, Ellipses, Omission of Ellipses).
“The following year  was a turning point.”...
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Parentheses and brackets are internal punctuation marks used to set off material that is nonrestrictive or, as in the case of mathematical and chemical expressions, to alert the reader to the special functions occurring within. Note: Excessive use of parentheses and brackets can make text challenging to read and understand....
Use parentheses to indicate supplementary explanations, identification, direction to the reader, or translation (see 8.3.2, Dashes, and 8.5.2, Brackets).
A known volume of fluid (100 mL) was injected.
The differences were not significant (P = .06).
One of us (B.O.G.) saw the patient in 2006....