Final answer:
When filling out the DD Form 553, if certain information is not available, one should write 'N/A' in the relevant blocks to indicate that the information is either not applicable or is unavailable.
Step-by-step explanation:
On the DD Form 553, which is a Department of Defense form, if information is not available for certain blocks, you would write 'N/A' in those blocks. 'N/A' stands for Not Applicable or No Answer, and it is used to indicate that the question does not apply to the particular situation or that the answer is not available at the time of filling out the form. This helps to maintain the integrity of the form by clearly showing that the field was not overlooked but rather that it was intentionally left blank because the information could not be provided.