What does (…, Optional) mean in API parameters?

What does the “Optional” mean in these parameters I keep reading?

index (number, optional): the new material index

Link to API page: Rhino - RhinoScriptSyntax

It means that you dont need to pass a index, because the default is “none” but if you like to set it you mist pass a integer.

Oh, I would think the “,” would be a value separator inside the parentheses like that.
Thx for the auick reply