ABP Framework Domain Services

The primary business logic is typically implemented in aggregates (entities) and the domain services in a domain driven design (DDD) solution. It is particularly necessary to create a domain service when; You implement a fundamental domain logic that relies on a few services (such as external services or repositories). The logic you need to use relates to multiple aggregates and entities, thus it doesn't fit in any of the aggregates in the right way. https://docs.abp.io/en/abp/latest/Domain-Services