Tiered link building boosts your site's authority by building links in layers....

https://www.4shared.com/office/BIObF2h6fa/pdf-72309-44204.html

Tiered link building boosts your site's authority by building links in layers. Start with Tier 1 links from trusted, relevant sites that point directly to your page. Then, support those with Tier 2 links that strengthen your Tier 1 sources