In DDD, tactical design refers to the process of designing the individual components of a software system to implement the domain model. It involves breaking down the domain model into smaller, cohesive and loosely coupled units of functionality called bounded contexts, aggregates, entities, value objects, and domain services. Tactical design is concerned with the design of these units and the relationships between them. It also involves selecting the appropriate patterns, strategies, and tactics to achieve the design goals while adhering to the domain-driven design principles.

The elements of tactical design in Domain-Driven Design (DDD) are as follows:

1. Bounded Contexts: Bounded contexts define a clear boundary around a specific part of the domain model and the ubiquitous language that goes with it.
2. Aggregates: Aggregates are clusters of related objects that are treated as a single unit of work.
3. Entities: Entities are objects that have unique identities and lifecycles and are mutable over time.
4. Value Objects: Value objects are objects that have no unique identity but are defined by their attributes.
5. Services: Services are operations or behaviors that do not naturally fit within a single entity or value object.
6. Domain Events: Domain events are important events that occur within the domain that other parts of the system may need to know about.
7. Domain Services: Domain services are operations or behaviors that apply to the domain as a whole rather than to a specific entity or value object.
8. Factories: Factories are used to create complex objects or aggregates that may require multiple steps or complex logic.
9. Repositories: Repositories are used to abstract the storage and retrieval of aggregates and entities.

Tactical design is called so because it focuses on the tactical decisions related to modeling the domain and implementing the business logic in code. Tactical design is the process of implementing DDD patterns, principles, and practices to create a flexible, maintainable, and scalable software system. It is an essential part of DDD that helps developers to organize their code and improve the quality of the software system. Tactical design provides a set of guidelines and best practices that developers can follow to create a domain model that is aligned with the business requirements and that can evolve over time.