[MichaelZuo]

Did you thoroughly read through the linked sources?

Because in source number 5, it mentions that some fraction of Class III submissions are cleared through the 510(k) process and not exclusively through the PMA process.

> In 2007, Congress asked the Government Accountability Office (GAO) to review the 510(k) process. The resulting 2009 GAO report described the 510(k) process as less stringent, faster, and less expensive than the PMA process and concluded that 66% of Class III submissions cleared through the 510(k) process in recent years were “implantable, life sustaining, or of significant risk,”

[seehafer]

> Because in source number 5, it mentions that some fraction of Class III submissions are cleared through the 510(k) process and not exclusively through the PMA process.

Yes, these are what are known as "pre-amendment" devices, referring to those types of devices that were legally marketed in the US prior to the 1976 amendment to the FD&C Act that gave FDA power to regulate medical devices. FDA was given power to require these devices require PMAs via rulemaking, and has been slowly (far too slowly, in the views of many) closing this loophole.

[MichaelZuo]

I'm pretty sure the GAO report is largely referring to Class III devices approved after 1976 in this section... under less restrictive rules of some kind.