Which of the following statements is true for a standard internal table?

The correct understanding of standard internal tables in ABAP is that their key is always non-unique. Standard internal tables allow for instances of the same key to exist multiple times within the table. This characteristic is particularly useful when working with collections of data where duplicate entries are permissible and often expected, such as transaction logs or lists of items where repetitions are common.

In practical applications, this flexibility enables developers to aggregate data without the need to enforce uniqueness, making standard internal tables suitable for tasks where accumulation or collection of data entries is required. Consequently, the use of unique keys is not relevant when dealing with standard internal tables, as their design inherently supports duplicate entries.

The other options would imply different behaviors not associated with standard internal tables: unique keys imply a singular instance for each key value in the table, and choosing between unique and non-unique would imply an optional constraint not present in standard internal tables. Leaving the key undefined does not align with the functionality of standard tables, as there is no key concept in a void state.