Three principal types of emission credits may also be traded under this regime: (a) the “emission reduction unit” (ERU); (b) the “certified emission reduction” (CER); and (c) the “removal unit” (RMU). Each ERU, CER, and RMU is an entitlement to emit the equivalent of one metric ton of carbon dioxide into the atmosphere. […] CERs arise from the “clean development mechanism” in Article 12 of the protocol.
Source: wiktionary