Explanation
The main difference between software development methodologies and systems development methodologies is that software development methodologies tend to focus on high quality code, such as functionality, reliability, performance and maintainability, while systems developmentmethodologies tend to focus on holistic issues, such as stakeholder needs, system boundaries, interfaces, trade-offs and lifecycle management.
Software development methodologies are usually applied to software-intensive systems or subsystems, while systems development methodologies are usually applied to complex systems that involve multiple disciplines and domains. It is not true that all current systems development methodologies are extensions of previous software development methodologies, as some systems development methodologies have different origins and foundations. It is not true that software development methodologies employ formal architecting techniques, while systems development methodologies tend not to, as both types of methodologies can use different levels of formality and rigor in their architecting approaches. It is not true that for software intensive systems, there is effectively no difference between system development methodologies and software development methodologies, as software intensive systems still require a broader and deeper perspective than software development methodologies can provide. It is not true that there is no difference between any methodology good for software development and any methodology good for systems development, as different types of systems may require different types of methodologies that suit their characteristics and challenges. References:
OMG-Certified Systems Modeling Professional - Model Builder - Advanced (OCUP2-ADV) Examination Guide Version 1.0, Section 4.5