This is the most basic format of the document, the representative of something in brackets option, you can also fill can not fill
There are similar | representation or symbol that can fill this, you can fill that, like
color: inherit | transparent | <color-value> | <color -keyword>
above represents a value with a certain type of <> the wrapped, many programming languages, too (template class)
there are similar
foo (a, [b = 3 ])
indicates if you do not pass the second parameter, the second parameter b, default value 3
then | and [] can be used in combination
[] may also be nested, such as
bar (a [, b [, c]])
says that if filled b, c can optionally pass
but did not pass if b, c can not pass
but really talking about it the moment I really can not think of something where formal introduced, after all, different places or slightly different syntax