The Código Especificador da Substituição Tributária (CEST) or in english, Tax Substitution Specifier Code is part of an ICMS rule defined by ICMS Convênio no. 92 of August 20th, 2015 established the standard and identification on products and goods that are ICMS-ST (Tax Substitution)
A List of products and goods that are subjected to this rule can be found in the Annex I to XXIX of this rule and in the CONFAZ Note S/No. from October 20th, 2015.
All Tax Contributor Companies issuing a nota fiscal selling or transferring goods and products that may be taxed to ICMS-ST or to ICMS collect anticipation listed in the Annex above, must inform the corresponding CEST (Código Especificador da Substituição Tributária) code in the xml of that respective nota fiscal, regardless the transaction or good that are subjected to ICMS ST or to ICMS collect anticipation.
SAP release the OSS Note 2236378 – LC – Convênio ICMS 92/2015 to cover this requirement on ERP
The configuration of CEST is done via J1BTAX, in the Menu Entities > CEST or you can use the SM30 > J_1BTCEST to maintain the CEST codes and J_1BTCESTDET to assign the CEST code by NCM or Material Number.