This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Some cats test positive on the coronavirus titer test, some do not. Those which have never been exposed to ANY form of coronavirus are called "seronegative." Those which have been exposed to some form of coronavirus are called "seropositive." The Winn Foundation-sponsored research felt the Cornell study was flawed because it used cats which had already been exposed to some form of coronavirus (were "seropositive") and then attempted to test the vaccine.
This is not to be confused with the terms "FIP negative" and "FIP positive" which are used by many labs to indicate that the coronavirus titer is less than (negative) or more than (positive) some predefined threshold level.