Brackets ( [ ] ) enclose optional elements. When several bracketed entries are stacked vertically, then you can select one (but no more than one) of these entries.
Braces ( { } ) indicate that you must select one (but not more than one) of the enclosed vertically stacked entries. If only one entry appears, then the braces serve as delimiters for repetition (see Ellipses in the next section).
Vertically stacked entries enclosed in vertical bars indicate entries that you may select one or more of. Any number of entries can be selected, but no entry may be selected more than once.