In both cases, the high-level design should be a complete view of the entire system, breaking it down into smaller parts that are more easily understood.
A high-level design document or HLDD adds the necessary details to the current project description to represent a suitable model for building.
The highest-level design should briefly describe all platforms, systems, products, services, and processes that it depends on, and include any important changes that need to be made to them.
In addition, there should be brief consideration of all significant commercial, legal, environmental, security, safety, and technical risks, along with any issues and assumptions.
The idea is to mention every work area briefly, clearly delegating the ownership of more detailed design activity whilst also encouraging effective collaboration between the various project teams.