Clarification Request on EBNF Syntax in C99 Grammar File

[rpjax]

I am currently using the C99 EBNF grammar file from the "examples/c99-grammar.iso-ebnf" directory for testing my parser generator algorithm and applying some grammar transformations. During this process, I came across a notation that I'm unfamiliar with.
Ex:

struct-or-union-specifier
    <missing '='> 
    | enum-specifier
    | typedef-name;

Could you please explain what <missing '='> represents?