Aha! Methinks a good read of Gen is required. Square Brackets are different to curvy ones in AIP.
4.2.1 Symbols and Parentheses
Words in parentheses “( )” indicate that specific information, such
as a level, a place, or a time, etc, must be inserted to complete the
phrase, or alternatively, that optional phrases may be used.
Words in square parentheses “[ ]” indicate optional additional
words or information that may be necessary in specific instances.
J