This part of ISO/IEC 14143 defines the fundamental concepts of Functional Size Measurement (FSM) and describes the general principles for applying an FSM method. This part of ISO/IEC 14143 does not provide detailed rules on how to 1) measure Functional Size of software using a particular method, 2) use the results obtained from a particular method, 3) select a particular method. The definition of FSM in this part of ISO/IEC 14143 is applicable when determining if a method for sizing software is a Functional Size Measurement Method. It does not prevent the development of various methods, but rather provides a basis for assessing whether a particular method conforms to FSM. This part of ISO/IEC 14143 is intended for use by those persons associated with the acquisition, development, use, support, maintenance and audit of software.