Medical electronic devices quality
The only 'grade' that is sometimes specific to medical components is encapsulation. Stuff that is going to be used "in people" has different encapsulation requirements than stuff that is going to be used "in a home"
Medical components have test, traceability and documentation requirements. Test, traceability and documentation typically require that the production and distribution channels meet test, traceability and documentation requirements.
For the Atmega 328, it may only be used with "an Atmel officer's specific written consent." If you want a medically qualified 328, you'll have to pay for the test, traceability and documentation. And the business insurance.
There is no defined grade for medical. There are only norms defining what a medical product must comply with.
Some manufacturer may have e a special product series of a part such as military, space or aviation grade, but you usually don't see those on the manufactures websites.
Most common parts have the text line you mentioned because of liablity.