A "Professional" camera is/was one that the manufacturers and distributors would:
1) "accessorize" with professionally oriented features and accessories;
2) market to professional photographers; and
3) support with enhanced repair and accessory resources.

This means that there are a wide variety of types of "professional" cameras. For example, a photojournalist requires one type (a Nikon F2?), whereas someone doing high volumes of photo-micrography might choose an Olympus OM body.

Was an FM3A a body that Nikon would support in the same way they would support an F5?