in AWSClientRuntime/Sources/Endpoints/Partition.swift [51:56]
func resolveEndpoint(region: String) throws -> AWSEndpoint {
let shouldUsePartitionEndpoint = region.isEmpty && !partitionEndpoint.isEmpty
let resolvedRegion = shouldUsePartitionEndpoint ? partitionEndpoint : region
let endpointDefinition = endpointDefinitionForRegion(region: resolvedRegion)
return try endpointDefinition.resolve(region: region, defaults: defaults)
}