The comment frequency in a code fragment relates to the number of comments in relation to the code size. A good level of comment frequency can significantly contribute to the maintainability of the software. Maintainability is a non-functional quality characteristic that refers to the ease with which a software system can be modified to correct defects, update features, improve performance or other attributes, or adapt to a changed environment. Comments in the code help developers understand the logic, purpose, and functionality of the code, which is crucial when modifications are required. This does not directly contribute to portability, usability, or performance efficiency, which are concerned with different aspects of the software's operation and user interaction.