There are two distinct life cycles at work in a project—the project life cycle, which encompasses all the project work including the systems development, and the systems development life cycle, which focuses on the product or products of the project. Many organizations make the mistake of trying to manage their project using only the SDLC. It is a mistake to assume that either the PLC or the SDLC is sufficient by itself—both are needed to successfully manage any project, but particularly one as complex as any IT project.
The PLC and the SDLC models have overlapping activities. It is not possible to completely separate the two any more than it is possible to delineate between the phases of either model. But thinking of the models as having distinct phase activities is helpful in understanding what the project manager and his team have to do in order to successfully complete the project on time, within budget, and to the customer's performance specifications.