<p>The possibility of using CNC machine tools as measuring systems brings several advantages. The main advantage is to increase the productivity of the production process through in-process intermittent measurement, which allows to save working time by eliminating intermittent operations. However, such measurement systems must meet the metrological requirements of the measurement task to ensure the credibility of the measured data. The assessment of the applicability of a CNC machine tool as a measurement system can be done by verifying conformity with specifications according to the ISO GPS, or by demonstrating the capability of the measurement system by methods that use specifical statistical approaches (e.g. ISO 22514–7:2021, MSA:2010, VDA 5:2011). Based on the result of the specification assessment, it is then possible to approve or reject the deployment of the relevant CNC machine in the measurement process. This paper critically analyzes the current state of the art in this field and presents a new approach to assessing the applicability and capability of CNC machine tools as length measurement systems. The analysis compares the measurement process on a three-axis vertical CNC machining center with the measurement process on a coordinate measuring machine. Based on the critical analysis, an original approach to properly understand and apply the concept of combined measurement uncertainty in assessing the applicability and capability of a CNC machine as a measurement system (MS) in the on-machine measurement process has been developed and presented in the paper.</p>