Syntax question: What is BORDER in 4.17. Text Markup?

[Jens Lechtenboerger]

Hi there,

the syntax for Text Markup such as *bold* at [1] specifies
PRE MARKER CONTENTS MARKER POST with
CONTENTS as BORDER BODY BORDER and
BORDER as “Any non-whitespace character.”

What is the role of BORDER here?  Does it really exist?

What is BORDER if CONTENTS should be a single character, e.g., in
the two strings “*x*” and “~*~”?  Are single characters forbidden?

Best wishes
Jens

[1] https://orgmode.org/worg/org-syntax.html#Emphasis_Markers