What is an effective way to define target customer segments in the payments industry?

Very vast question, i doubt you would find something that is equivalent to a generic descriptor to help you with this.

Below is a generic (rough) model of looking at a payments ecosystem with regards to the target consumer market. No one solution fits all, and not necessarily top-tiers are the most expensive and lower tiers the most cheapest, but this does give some form of semblance.