What is rel="external nofollow"? Does this affect seo?

Why all blogs and news websites using rel="external nofollow"?

I know rel="nofollow" but What is rel="external nofollow"?
I used rel="external nofollow" in my blog and news pages. Does this affect seo?
If any other alternative rel="...." attribute is there?

If you know the answer means kindly share your ideas. Thanks..

It does not affect SEO, "external nofollow" are being handled by Google like the normal "nofollow", which means that link juice will not be passed.

There are other rel attributes like

• canoncial
• nofollow
• author
• alternate
• stylesheet

and "external" which just indicates that the "document" is located on a different website.