ISO/IEC 14143-6:2006 provides a summary of the FSM (Functional Size Measurement) related International Standards and the relationship between;

• the ISO/IEC 14143 series FSM framework International Standards that provide the definitions and concepts of FSM and conformance and verification of FSMMs (Functional Size Measurement Methods), and
• the ISO/IEC standard FSMMs, i.e. ISO/IEC 19761, ISO/IEC 20926, ISO/IEC 20968 and ISO/IEC 24570.

An FSMM is a software sizing method that conforms to the mandatory requirements of ISO/IEC 14143-1.
ISO/IEC 14143-6 also provides a process to assist users to select an FSMM that meets their requirements. It also gives guidance on how to use Functional Size (FS).
FSMMs include, but are not limited to, ISO/IEC 19761, ISO/IEC 20926, ISO/IEC 20968 and ISO/IEC 24570, as well.
Recommending a specific FSMM is outside the scope of ISO/IEC 14143-6.