High-level design explains the architecture which would be used for developing the software product. The architecture diagram provides an overview of an entire system, identifying the main components that would be developed for the product and their interfaces. The HLD uses possibly non-technical to mildly-technical terms which should be understandable to the administrators of the system. In contrast Low level Design further exposes the logical detailed design of each of these elements for programers.
Preliminary design - In the preliminary stages of a software development the need is to size the project and to identify those parts of the project that might be risky or time consuming.
Design overview - As the project proceeds the need is to provide an overview of how the various sub-systems and components of the system fit together.
In both cases the high level should be a complete view of the entire system breaking it down into smaller parts that are more easily understood. To minimize the maintenance overhead as construction proceeds and the lower level is done it is best that high level design is only elaborated to the degree needed to satisfy these needs.
High Level Design Document
High Level Design Document is the acronym for HLDD. A high-level design document is to add the necessary detail to the current project description represent a suitable model for coding. These documents include a high-level architecture diagram depicting the structure of the system, such as the database architecture, application architecture (layers), application flow (Navigation), and technology architecture. 
A high-level design provides an overview of a solution, platform, system, product, service, or process.
Such an overview is important in a multi-project development to make sure that each supporting component design will be compatible with its neighbouring designs and with the big picture.
The highest level solution design should briefly describe all platforms, systems, products, services and processes that it depends upon 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, 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.
Today, most high-level designs require contributions from a number of experts, representing many distinct professional disciplines.
Finally, every type of end-user should be identified in the high-level design and each contributing design should give due consideration to customer experience.
- "Designing in a Big Way - a practical guide for managers", High Level Designs Ltd,
- High Level Design Document sample format