What does Retail Impact Assessment mean in a planning context?

An assessment undertaken for an application for retail use (normally on developments over 2,500 square metres gross floorspace, but they may occasionally be necessary for smaller developments, such as those likely to have a significant impact on smaller centres) on the impact of the proposal on the vitality and viability of existing centres within the catchment area of the proposed development. The assessment includes the likely cumulative effect of recent permissions, developments under construction and completed developments.