First of all we have to know why choose tier back linking? 1.own control 2.quality site 3.contextual back linking 4.avoid search engine penalties such as(penguin, panda) Then why have to be tier back linking, Tier-1 is to direct link to your money website by (PBN, EXPIRED DOMAINS.) Tier-2 is to make loop of your tier 1 sites that is again and again it get advertising your sites other than tier 1 advertise areas(i.e. back link the tier 1 sites). Tier-3 is to make loop of the tier 2 sites for advertise.