What is Accordion's open prop?

[erfanium]

I use reactstrap@v9.0.0

This is the type of Accordion component props

export interface AccordionProps extends React.HTMLAttributes<HTMLElement> {
  tag?: React.ElementType;
  cssModule?: CSSModule;
  flush?: boolean;
  innerRef?: React.Ref<HTMLElement>;
  open: string | string[]; // <- this
}

What is that required open key? I didn't find anything about this in the docs

[erfanium]

I find a usage from here

But missing in docs😁