Tier 1 consists of the websites that pass link-juice directly link to your website. These are high-quality backlinks that are created to increase the website SEO score. This is the most important tier.
Resources include: Web 2.0, Blog posts, Article Directory
Tier 2 consists of such websites that pass link-juice directly to Tier 1 websites. Websites in Tier 2 get backlinks from the websites in tier 1.
Resources include: Articles and content on HubPages, blogspot, or other official blogs
This third tier of backlinks will be pointing directly to your Tier 2 backlinks. The goal of tier 3 backlinks is to improve the index rate of your tier 2 backlinks. The majority of your tier 3 backlinks will not be indexed by Google so you should create enough that you will have at least a few backlinks indexed per tier 2 backlinks on average
Resources include: Wikilinks, Comments, bookmarks
All created backlinks are sent to 3 indexers in Drip mode. The best way to index links is by dripping. Dripping indexes links slowly and they are indexed 7-30 days after delivery. By crawling slowly, the link building strategy becomes more secure and nature from a search engine perspective. It is never good to have hundreds or thousands of backlinks indexed instantly. The slower the process, the safer and more natural it will be.