In human text, square brackets are often used within quotes to indicate uppercase/lowecase initial change (when around a single letter), replace or clarify indirect and implicit references, or to add clarifications like [sic] (to indicate an error in the original). In general, it specifies a difference to the exact original.
For example, if I said "I like apples. I know it is said that eating one a day keeps the dogtor away.", one can quote me precisely using "[I]t is said that eating one [apple] a day keeps the dogtor [sic] away."
You can find this in several style guides, I believe.