It should be added that on occasion the distinction between a "note", requiring round brackets, and a "definition", requiring curly brackets, is somewhat moot.
(d) Hints and expansions
Entries in square brackets are hints or expansions to make the quoted text more intelligible, and can be thought of as being preceded by the qualifications "that is", "in other words", or "what Spenser appears to mean is". For example:
in his help > [to help him; in his armoury]
Such entries can be mentally substituted for the quoted word or phrase in order to aid comprehension.
Sometimes square brackets are employed in other sorts of definitions to indicate words which should be understood. For example:
mask > {Disguise [herself]; take part in a masque or masquerade}
and:
time > [her] term of apprenticeship
(e) Explanations of character-names