SAP NetWeaver AS ABAP Release 740, ©Copyright 2014 SAP AG. All rights reserved.
ABAP Keyword Documentation → ABAP Glossary →Secondary table keys
Up to 15 secondary table keys can be created for each internal table. Both sorted keys and hash keys are possible. A secondary table index is created internally for each sorted secondary key. In the case of accesses to internal tables, you must specify the table key to be used.