Tiered link building boosts your website’s authority by using multiple layers...

https://wiki-nest.win/index.php/What%E2%80%99s_a_Good_Tier_1_Link:_Guest_Post_vs_Niche_Edit_vs_PR_Mention%3F

Tiered link building boosts your website’s authority by using multiple layers of backlinks. Tier 1 links point directly to your site, while Tier 2 links support those Tier 1 sources