Maintainability represents the degree of effectiveness and efficiency with which a product or system can be modified by the intended maintainers (ISO 25010). Maintainability incorporates such concepts as changeability, modularity, understandability, testability, and reusability. Maintainability is responding rapidly to market conditions and keeping IT costs under control. The Maintainability of an application is a combination of compliance with good coding practices, the homogeneity with which coding rules are applied across an application, and compliance with architectural rules.
The CISQ Automated Source Code Maintainability Measure is composed from 20 critical violations that reduce the maintainability of a software application. The CISQ Maintainability measure produces a quality score based on the count of violations discovered in the software that can be translated into a density measure when divided by software size.
You can also download the OMG® standard here: http://www.omg.org/spec/ASCMM/