Premium is not calculated on each session, but on the whole portfolio of the ceding company.
session means the liability of insurance company is reduced/eliminated and reinsurer company is increased/established.
now in non – proportional treaties single policies are not ceded nor class of policies (like all marine policies etc), in these non-proportional policies re-insurer takes liability to bear losses above certain limit for company as a whole and not individual policy or small group of polices.
for example re-insurer will bear claims if total business claims crosses 500 crore. so re-insurance company will bear losses not on case to case basis but for business as a whole