If it's ET you're being tested for or a similar blood condition, secondary would be down to either an external factor (chemicals etc) or it being secondary to another issue within your body (spleen removal, inflammatory issues etc). Primary would simply mean your body is producing too many platelets without anything else being a factor. 

I had secondary polycythaemia (too many red blood cells. My platelets were fine) years ago, which was caused by chronic dehydration. So mine wasn't considered a cancer being secondary, whereas had my body just been creating all those blood cells on its own, without any outside factors, it would have been a cancer. Mine cleared up once i ceased alcohol, and drank water regularly. So secondary clears up once/if the underlying cause is treated.