Available information, e.g. from conceptual engineering or from a feasibility or rehabilitation study serves as input for basic engineering. Within the basic engineering, maybe in an iterative process, the plant is developed.
Basic engineering includes preparing drawings like layout, sections and views as well as technical specifications. A clear concept for the plant is engineered and the overall plant configuration is available.
Major equipment with long delivery times may be specified in detail already during this phase in order to speed-up the execution of a project and have a more solid basis for cost estimations.
During the detail engineering, documents from basic design are updated for construction.