Minimum standard - SML3
Expected standard - SML4
Excellent standard - SML5
SML1 - Initial usability: The software was not designed with extensibility in mind, so there is either no ability to extend or modify program behaviour, or it is very difficult to do, even for usages similar to those of the software core design; execution parameters cannot be changed. There is no, or limited, availability of the source code; the logical flow of code may be hard to follow, with little to no cohesion.
SML2 - Use is feasible: There is some consideration to extensibility, but that may only exist for a limited number of use cases, through use of methods such as object-oriented design or other tools which provide logical cohesion. Some extensibility is possible through configuration changes; isolation of configuration parameters and constants in clearly identified sections of source code; and/or limited opportunity for software modification.
Where source code is available, there is evidence that there is effective use of programming practices designed to enable use, such as object oriented design.
SML3 - Use is possible by most users: Future extensibility is designed into the system for a moderate range of use cases. The procedures for extending the software are defined, whether by source code modification or through the provision of some type of extension functionality (e.g., callback hooks or scripting capabilities). Where source code modification is part of the extension plan, the software is well-structured, has a moderate to high level of cohesion, and has configuration elements clearly separated from logic and display elements.
SML4 - Software is usable: The extensibility capability for the software is well defined, broad range of use cases, providing many points of extensibility. A detailed extensibility plan is publicly available and is sufficient to allow an experienced developer to become familiar with the project to extend the software in a reasonable amount of time. Documentation should include clear information about the range of use cases to which the software can be extended as well as potential limitations on expansion. There is evidence that the software has been extended and applied to a context to the original. This extension may have been done by another group or project, using extension documentation, but may have involved ad hoc and substantial assistance from the original development team.
SML5 - Demonstrable usability: There is evidence that the software has been extended externally by users outside of the original development group using existing documentation only. There is a clear approach for modifying and extending features across a in multiple scenarios, with specific documentation and features to allow the building of extensions which are used across a range of domains by multiple user groups. There may be a library available of user-generated content for extensions and user generated documentation on extension is also available.