A deep link is essentially a link that points to a page that’s deep within your website’s navigational structure, if a link points to a product page or an internal page within your hierarchy that isn’t your Homepage, this may be considered a deep link.
A lot of industry SEO’s believe that an external link that links to a page deep within your navigational structure is considered a better link, as the juice is flowing directly to your landing pages, rather than your Homepage. A homepage external link will the dilute the juice to your child pages. In this process, the diluted juice isn’t as powerful as if it were a direct link.
Webmasters, bloggers and the less technical folks may link directly to the Homepage in their posts when referencing a certain company, whereas, for SEO and most importantly, user experience, it makes sense to include a deep link to the more appropriate page on the other domain – otherwise, the user won’t know where to go after they have landed on the Homepage.
In essence, the deeper the link, the better it is (I think). If you can get a lot of links to a specific section of your site rather than your Homepage, fabulous. However, it’s always good practice to power your domain beforehand, therefore, the juice is diluted to all of your pages. Plus, this helps increase website authority which is always useful.