Primary vs. Secondary Codes For Body Parts

I am wondering how you code for body parts where a primary or secondary modification is listed, such as :

Coding for an initial visit of left knee osteoarthritis, but we've seen them before for osteoarthritis of the right shoulder.

Does this mean I code the knee OA as secondary? Or is it primary because this is the body part we are seeing the patient for today?? And what does secondary actually mean??

Thanks :)