I have a module that needs to tell me whether a customer is present in more than one country in each region. For example, Customer A is only present in United States under AM region. This means that it is not an inter-region customer in AM. But in EU region, it is present in United Kingdom and Italy, so it is an Inter-Region customer in EU. Please see table below for visual reference. I can't figure out a formula for the Inter-Region column to get this requirement.
To clarify consider moving the assignment of customer > country out of a List and into a Module. Having customers roll up under multiple parents within the same list is not a true hierarchical relationship and may cause confusion and inefficiencies in the future. Having the assignment in a module will allow you to capture and track the ever evolving relationship between customer and country.