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