For example: Jean Sibelius is not considered a violinist/composer, despite the fact that he played the violin, and neither is Jascha Heifetz, even though he wrote several cadenzas and transcribed showpieces.
For instance, Jean Sibelius is not categorized as a violinist/composer despite his proficiency on the violin, similarly to Jascha Heifetz who was a renowned violinist but did not compose in the traditional sense.