Final answer:
The line 'By direction of the commanding officer' is added when someone is signing on behalf of the commanding officer, indicating delegated authority.
Step-by-step explanation:
The line "By direction of the commanding officer" is added to the signature block when an individual is acting on behalf of the commanding officer. This circumstance usually occurs when the commanding officer has officially delegated their authority to another person to sign documents or carry out specific tasks on their behalf. It is not specific to the classification of the document, whether it is for internal use, or related to routine administrative tasks. Adding this phrase ensures that recipients understand that the person signing the document is doing so with the authority of the commanding officer, and is not necessarily the commanding officer themselves.