Proposal to provide guidance for interpretation of “Type of REESS” (Rechargeable Electrical Energy Storage System) as defined in UN R100 para. 2.48. The document establishes that differences in REESS should be evaluated from the perspective of substantial impact on safety performance. It describes key elements for assessing impact on safety within each essential aspect: manufacturer responsibility, cell chemistry/capacity/physical dimensions, cell number/connection/support, casing construction/materials/dimensions, and ancillary devices. The guidance provides examples of changes not causing or potentially causing significant differences, including production location changes, multi-sourcing of cells, and process improvements.