I personally have no experience working with PMOA for combination devices. But the best way to understand what is the PMOA is by seeign the two possible designation. And then evaluate if it were to go through the FDA process with the two possible designations individually, which one will match the intended therapeutic effect or action. This helps to make the determination because if it the portion that needs to be evaluated by the FDA through the end to end process.

http://www.meddeviceonline.com/doc/how-to-determine-a-combination-product-s-primary-mode-of-action-pmoa-0001