Thank you so much Andrea!
Basically a current negative coggins proves a clean bill of health--meaning no infectious diseases. It has no bearing on physical health or soundness. It does document markings and other physical attributes. Current negative coggins are required by some states to sell, transport, or board a horse. Hope this helps.
Thank you so much Sophie! This really helped me
I found this definition :
It is a serological immunodiffusion test for the diagnosis of equine infectious anemia especially in horses by the presence of antibodies to the causative retrovirus.
If you need more information about it, I advise you to go to : https://en.wikipedia.org/wiki/Equine_infectious_anemia it seems to be a serious article about equine infectious anemia.