For the uninitiated, the FDA faced criticism initially by being too slow to authorize PCR tests for SARS-CoV-2 and issued a much more broad EUA for serological tests. However, serological tests need to be run on known positive and negative patients (already tested) to validate their accuracy. Because infection with SARS-CoV-2 is relatively uncommon, and because the EUA is broad, tests that offer the statistical value of a coin flip have been authorized and used. This is a problem if you’re trying to make health decisions.
So, the FDA is trying to advise people on this risk by providing tools to judge the performance of serological tests to help clear up the confusion. This is because many tests have rated performance based on a very small number of people and the error bars are relatively large.