Mnemonics

"box", "array"
-- main use is for arrays
-- also to create reduce meta operators

Used alone

Used as a brace pair or quote pair

[] in a term treats it as an array
[] instead of () around list values creates an Array object and can explicitly make a list from a single value
[] around an operator creates a reduce meta operator
[] can be used for several radix-related purposes when defining literals
[] use of an empty pair of boxy brackets after an array allows it to be interpolated into double quoted strings.

Operators containing this character

; interpolates a list into a semicolon list, which is just a reduce meta operator but worth noting.

When used inside regexes

[] groups without capturing

Special Variables

As an adverb

[] after an adverb can assign an Array object to an adverb
[] where an adverb would appear creates an identifier extension

Other Uses

Hyper operator and meta operator restrictions

Old, deprecated, or other language uses