High Level Design (HLD) and Low Level Design(LLD) Documents

High Level Design Document: HLD is the design document convert the user requirement into the detailed design of Application. In HLD document, application is divides into different different modules. After that each modules divides into micro modules and then each micro modules take at a time to work over it.

This document include the System Architecture, Database Architecture, Structure of the System, Application Architecture, Application flow, Technology Architecture
and the Security Architecture.
This document very useful for the developer to understand the flow of the application. In this phase entry criteria is the Requirement Document and the HLD is the exit criteria.

Low Level Design Document: LLD is the design document prepared when the modules design of HLD will be broken into the micro level. LLD document contains the  date structure required software architecture, source code and performance algorithm.

LLD is the stage where actual software component are designed.  It also contains the detailed functional logic of the software, Database Table with element with type and size, dependency, input and output between interfaces.

Entry Level is HLD documents, and the Unit Test plan is the exit level.