Minimum standard - SML2
Expected standard - SML3
Excellent standard - SML5
SML1 - Initial usability: Software application is buildable and runnable. Unit testing has been performed and code coverage metrics obtained.
SML2 - Use is feasible: Software application has been built, tested and run. Testing includes stress testing, functional testing and end to end testing.
SML3 - Use is possible by most users: A release candidate has been demonstrated, tested and validated in a relevant context.
SML4 - Software is usable: A release candidate has been "qualified" through test and demonstration (meets requirements) and successfully released.
SML5 - Demonstrable usability: A production system has been tested and validated through successful use of the application.