Explanation
A unique secondary key is a type of secondary key that ensures that the key combination of all the rows in a table is unique. A unique secondary key has two purposes: firstly, to speed up access to the table, and secondly, to enforce data integrity1.
It is created with the first read access of a table: This is true. A unique secondary key is created when an internal table is filled for the first time using the statement READ TABLE or a similar statement. The system assigns a name and an index to each row of the table based on the key fields23.
It is updated when the modified table is read again: This is false. A unique secondary key does not need to be updated when the internal table content changes, because it already ensures data uniqueness. The system uses a lazy update strategy for non-unique secondary keys, which means that it delays updating them until they are actually accessed23.
You cannot do any of the following:
It is created when a table is filled: This is false. As explained above, a unique secondary key is created only with the first read access of a table23.
It is updated when the modified table is read again: This is false. As explained above, a unique secondary key does not need to be updated when the internal table content changes23.
References: 1: Improving Internal Table Performance Using Secondary Keys - SAP Learning 2: [Secondary Key - ABAP Keyword Documentation - SAP Online Help] 3: [Secondary Table Key - ABAP Keyword Documentation - SAP Online Help]