High-level design (HLD) explains the solution’s major parts and how they work together. Low-level design (LLD) specifies enough detail to implement those parts consistently. These are useful working terms; organizations do not share one mandatory document template.
Follow one decision through both levels
For a returns portal, HLD identifies the portal, returns service, order system and warehouse integration. It states which system owns each record and which interactions are asynchronous.
LLD describes a particular interaction: request fields, validation, state transitions, duplicate handling, database constraints and error responses. An engineer should be able to implement it, and a tester should know the expected behavior.
