Post marketing studies are usually assessed per the staetd elements addressed by the labelling along with degree and severity of adverse events that MAY be assocoted with use.
It is a way to pik up any inconsistencies that may be useful with changing, updating or adding/correcting labelling and, or positioning the drug for a potential use in addition to its initial approval.
Full FDA approval is a relative process and any additional information found with post marketing will be corrected/updated per outcome of trial(s)