Square brackets are used to indicate additions, deletions, or changes to someone else's words when you are quoting what they said or wrote.
For example, if I say:
"Every other day I eat peanut butter cookies, figs, candy canes, and an assortment of tree bark because they are delicious."
And someone quotes my statement in this question using square brackets: "Is it true that '[e]very other day [you] eat peanut butter cookies [...] because [you think] they are delicious'?".......
"[e]" has square brackets because it's changed from an uppercase letter to lower case letter;
"[you]" has square brackets because it is a completely different word;
"[...]" indicates that there were a bunch of words here that have been removed;
"[you think]" is in square brackets because it's something added to my statement;
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"Coming back to where you started is not the same as never leaving." -- Terry Pratchett, A Hat Full of Sky
Love transcends all.