In practice, which Life Cycle model may have more, fewer or different levels of development and testing, depending on the project and the software product. For example, there may be component integration testing after component testing, and system integration testing after system testing.

The V-Model is specifically designed to have testing levels that mirror development levels, but it allows for customization based on project needs. Additional testing levels like component integration and system integration can be inserted as needed. Waterfall has fixed sequential phases, RAD focuses on rapid prototyping, and Spiral is risk-driven with iterative cycles; all are less flexible about testing level insertion than V-Model.