Collection Groups in your Aria instance allow you to support multiple payment scenarios for different payment methods and currencies, either via single or multiple Payment Gateways. When an account or its Billing Group is associated with a Collection Group, Aria facilitates payments (and related financial transactions) for services to the specified Payment Gateway.
For most business use cases, associating an account with a single Collection Group is sufficient. However, if an account chooses to pay for different services with different payment methods, that account would require multiple Billing Groups. Depending on your Payment Gateway configuration, those Billing Groups might require different Collection Group associations.
A simple example is an account wants to pay for one service with Credit Card A, and another service with Credit Card B. One of your two Payment Gateways supports Credit Card A; and the other Payment Gateway supports Card B. The account requires two Billing Groups, each associated with a different Collection Group: one associated to the Card A Payment Gateway, and the other associated to the Card B Payment Gateway.