Proposal prepared by the REC expert group for the 01 series of amendments to the UN Regulation on uniform provisions concerning the approval of Retrofit Emission Control devices for heavy-duty vehicles. The text includes a revision in the definitions of the REC devices, stricter limit values (reduction levels) for REC systems and transitional provisions for the sale of REC systems that meet the requirements of the REC Regulation in its original form. To assure coherence with the change in the definitions, the text also includes a revision of the test sequence for particulate reduction and the equivalence matrix for the Regulation No. 49 Standard series.