Bill and Ron gave you good information. Think of the PMA as simply a
statement that the part is an approved substitute for the part from the
OEM that is being replaced.

One minor issue - a company can make a part and get an STC without
getting the PMA. It can then be installed *by the manufacturer* but it
can not be "offered for sale for installation on a certificated
aircraft" by someone else. For example, when GAMI was working on their
injectors, the FAA would not accept a PMA application until the STC had
been issued. So for a time they had the STC but not the PMA. They
could make the injectors and sell them to you, but THEY had to install
them. Once they got the PMA as well, then they could just mail them to
your local A&P for installation.

jmk