That's easy.
Inside quotes, square brackets are used to enclose words that were not actually said, but which are a shorter substitute for the actual words spoken.
eg
A witness in court may say
"When I opened the door, that ******* ******* forced his way past me into the house"
That might then be reported as:
"When I opened the door, [the accused] forced his way past me into the house"
Round brackets would be used only if what is enclosed were words actually spoken by the person being quoted.