hi,
When we use key internal table it faster the searching process at the read process.So the key may be unique or non unique.You can specify whether the key should be UNIQUE or NON-UNIQUE. Internal tables with a unique key cannot contain duplicate entries. The uniqueness depends on the table access method.
The user-defined key can contain any columns of the internal table that are not references or themselves internal tables. Internal tables with a user-defined key are called key tables. When you define the key, the sequence of the key fields is significant. You should remember this, for example, if you intend to sort the table according to the key.
Internal tables (SAP Library - ABAP Programming (BC-ABA))
ABAP Internal Table Secondary Key Performance comparison | ABAP Help Blog