In SAP S/4HANA, the material number field length was extended from 18 to 40 characters.This change was first implemented in SAP S/4HANA, on-premise edition 1511 and affects this release and higher releases.
Extending the material number on the database means that the field length of the MATNR field on the database has been extended from 18 to 40 characters. This has been done in all tables (and all fields within the tables) in which a material number can be stored.
MP's S/4HANA 