Railroad diagrams - what do the double bars on the ends mean?

Question

In the json site, the number and string diagrams are book-ended by double bars.

What do the double bars mean?

_{(source: json.org)}

_{(source: json.org)}

Answer 1

• One bar: whitespace can be inserted between pairs of tokens.
• Two bars: no whitespace allowed between pairs of tokens.

Answer 2

Single bars denote an element that is a "first-class" element (e.g. object, value), double bars delimit an element that is a subcomponent of another.