Hello @etb2, Hope this answers your question.

"Primary polycythemia is caused by an abnormality of the cells in the bone marrow that form red blood cells. Secondary polycythemia is caused by a disorder originating outside of the bone marrow that causes overstimulation of the normal bone marrow, leading to an overproduction of red blood cells."
--- An Overview of Secondary Polycythemia - An Overproduction of Red Blood Cells Caused by Health Disorders: https://www.verywellhealth.com/secondary-polycythemia-copd-complications-914682

There is a discussion that you may want to join post any questions you might have:
--- Polycythemia: Primary or Secondary?: https://connect.mayoclinic.org/discussion/polycythemia-primary-or-secondary/