In the classic Software Development Life Cycle (SDLC) process, Data Models are typically initiated, by model type, at key process steps and are maintained as data model detail is added and refinement occurs.
The Concept Data Model (CDM) is, usually, created in the Planning phase. However, creation the Concept Data Model can slide forwarded or backward, somewhat, within the System Concept Development, Planning, and Requirements Analysis phases, depending upon whether the application being modeled is a custom development effort or a modification of a Commercial-Off-The-Shelf (COTS) application. The CDM is maintained, as necessary, through the remainder of the SDLC process.
The Logical Data Model (LDM) is created in the Requirement Analysis phase and is a refinement of the information entities of the Concept Data Model. The LDM is maintained, as necessary, through the remainder of the SDLC process.
The Physical Data Model (PDM) is created in the Design phase to facilitate the creation of accurate detail technical designs and actual database creation. The PDM is maintained, as necessary, through the remainder of the SDLC process.