When should I use rel=noreferrer?

I have to link some other external sites.

I know when to use nofollow. But I am not clear when I should use rel=noreferrer.

In short, the noreferrer link type hides referrer information when the link is clicked. A link with the noreferrer link type looks something like this:

<a href="http://www.example.com" rel="noreferrer">Click here for more info</a>

If someone arrives at your site from a link that uses this link type, your analytics won't show who refered that link. Instead, it will mistakenly show as direct traffic in your acquisition channels report.

If you have an external link to someone else's site you don't trust and you want to hide referrer information then you can combine both and use

<a href="http://example.com/sample_page/" rel="noreferrer nofollow">Other Domain Link</a>

I advise you to use nofollow links for the following content:

• Links in comments or on forums - Anything that has user-generated content is likely to be a source of spam. Even if you carefully moderate, things will slip through.
• Advertisements & sponsored links - Any links that are meant to be advertisements or are part of a sponsorship arrangement must be nofollowed.
• Paid links - If you charge in any way for a link (directory submission, quality assessment, reviews, etc.), nofollow the outbound links