Sales Volume
You would get a different picture of the word tier from Amazon's compensation plan, where it refers to your sales volume.  (The following numbers don't reflect Amazon's actual compensation plan, but they serve as an example.)  On the first $500 worth of merchandise you sell, you receive a 10% commission.  Up to $500 is the first tier.  The second tier is for sales up to $1500.  If you manage to sell between $500 and $1500, you will earn 11% commission.  Each tier is an additional range of monthly sales in which you earn a higher commission (or receive other forms of compensation).

The Problem
The confusion arises when affiliate programs emphasize that they run a 2-tier compensation plan in their affiliate recruiting efforts.  In most cases, this refers to the downline levels definition, and participants can expect to receive compensation based on the performance of affiliates they introduce to the program.

Occasionally, a program makes this claim when referring to sales volume tiers.  This isn't inaccurate, but it is misleading.  The managers of these programs are almost never intentionally trying to mislead.  More often, they have thoughtfully crafted a compensation plan that rewards affiliates who perform well, and they want to announce that feature to all potential affiliates.

Who's Right?
You could make an argument for either definition of tier.  If you're trying to communicate with other people in the affiliate industry, though, you need to give strong consideration to which meaning is generally accepted by most people.  That is the downline levels definition.   Clay Mabbitt