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However, if one performs a validation in terms of the dimensions of the term software quality according to ISO/IEC 9126-1, from the customer's point of view the requirement can be summarised to 6 main criteria.

Main criteria	Sub-criteria
Functionality	Suitability, correctness, interoperability, compliance, safety
Reliability	Maturity, fault tolerance, availability, recover-ability,
Usability	Comprehensibility, learnability, usability, attractiveness
Efficiency	Time behaviour, resource consumption
Maintainability	Analysability, changeability, stability, testability
Portability	Adaptability, installability, conformity, interchangeability, compatibility

A total of 21 sub-criteria specify the importance of the main criteria. The first 4 criteria are extremely important for the end user of the application software. The latter two criteria are significant for the software producer.

Functionality is to ensure that the requirements of the specification are met. Reliability of the software product means its ability to fulfill its task within the framework of certain boundary conditions. In any case, good usability requires software that is easy to understand with a correspondingly comprehensible method, so that it is easy to learn and master. The efficiency of a solution refers to all the resources, including human resources, that are necessary for it to come together and during its operation. Good maintainability requires clear diagnosis when errors occur, ease of change when errors are detected, and little effort to test the success of the change. Probability is enshrined in IEC 61131 as a primary goal and is intended to make a program code usable for a new application.