In what cases will HTTP_REFERER be empty

I know it's possible to get an empty HTTP_REFERER. Under what circumstances does this happen? If I get an empty one, does it always mean that the user changed it? Is getting an empty one the same as getting a null one? and under what circumstances do I get that too?

It will/may be empty or partial when the enduser

• entered the site URL in browser address bar itself.
• visited the site by a browser-maintained bookmark.
• visited the site as first page in a new window/tab/session, in some browsers.
• clicked a link on a page having restrictive <meta name="referrer"> tag.
• clicked a link on a page having restrictive Referrer-Policy header.
• clicked a link having rel="noreferrer".
• clicked a link in an external application (i.e. not a webbrowser, e.g. Flash).
• switched from a https URL to a http URL.
• has security software installed (antivirus/firewall/etc) which strips the referrer from all requests.
• is behind a proxy which strips the referrer from all requests.
• visited the site programmatically (like, curl) without setting the referrer header (bots!).