IBM Professional Series User Manual page 178

• The use of braces denotes choices, one of which must be
selected.
Example:
The representation:
indicates that you must specify either A, or B, or C. If a
list of choices is enclosed by neither brackets or braces,
it is to be treated as if enclosed by braces.
The use of brackets denotes choices, one of which may
be selected.
Example:
The representation:
indicates that you may enter A, B, or C, or you may
omit the field.
In instances where there are nested braces or brackets
on the text lines, the following rule applies: nested
operand selection is dependent upon the selection of the
operand of a higher level of nesting.
Example:
Level 1 Level 2 Level
3
[filename [fi letype [filemode]]]
where the highest level of nesting is the operand that is
enclosed in only one pair of brackets and the lowest level
of nesting is the operand that is enclosed by the
maximum number of brackets. Thus, in the previous
example, the user has the option of selecting a file by
Chapter 7. VM/PC Commands 7-5