HTTP Referer

The Referer HTTP request header contains the absolute or partial address from which a resource has been requested. The Referer header allows a server to identify referring pages that people are visiting from or where requested resources are being used. This data can be used for analytics, logging, optimized caching, and more^[1].

Referer: https://developer.mozilla.org/en-US/docs/Web/JavaScript
Referer: https://example.com/page?q=123
Referer: https://example.com/

主要是以下三种场景，会发送Referer字段^[2]。

用户点击网页上的链接。
用户发送表单。
网页加载静态资源，比如加载图片、脚本、样式。

Referrer Policy

The original header name Referer is a misspelling of the word "referrer". The Referrer-Policy header does not share this misspelling.

Referrer-Policy: no-referrer
Referrer-Policy: no-referrer-when-downgrade
Referrer-Policy: origin
Referrer-Policy: origin-when-cross-origin
Referrer-Policy: same-origin
Referrer-Policy: strict-origin
Referrer-Policy: strict-origin-when-cross-origin
Referrer-Policy: unsafe-url

Also, it's possible to set it in HTML,eg:

<meta name="referrer" content="origin" />
<a href="http://example.com" referrerpolicy="origin">…</a>
<a href="http://example.com" rel="noreferrer">…</a>

Referer and HTTPS

[1] ttps://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer

[2] ttps://www.ruanyifeng.com/blog/2019/06/http-referer.html

[1]

[2]