This reads so confusing. Are "Tiers" and "Chapter Ranks" not the same thing?
You can read it like this.
Tier 1 = The first reward
Tier 2 = The second reward
Tier 20 = The 20th reward
etc...
This is not the same as Chapter Rank, because the Rank can only be earned through playing and earning Chapter XP. Whenever you rank up you will unlock the next reward.
If you Unlock Tiers by purchasing them directly, through the Chapter Booster or by completing the above mentioned week you will unlock the next X rewards but you will not get a higher Rank.
A higher rank is only available through gameplay and completing weekly challenges.
The Bronze, Silver and Gold Dogtag are tied to these ranks as well.