DD Form 2861 at a glance
What it is: DD Form 2861 is a Cross-Reference for the name, number, or subject under which this form is filed.
A filled-in DD Form 2861 example: a clerk fills in one cross-reference caption

How to fill out DD Form 2861, box by box
More rules for DD Form 2861
Prepare Cross-Reference sheets when (1) a document contains more than one name, number or subject under which it should be filed, and (2) sufficient copies of the record itself are NOT available for this purpose.
DD Form 2861 replaces Optional Form 21, which is obsolete.
Cross-references should be sufficiently descriptive to make them completely clear as to the documents they refer to.
If your situation is different
SINGLE CROSS-REFERENCE
For a single cross-reference, select the first sheet from the pad and enter the cross-reference caption in the open space.
TWO CROSS-REFERENCES
For two cross-references, select the first two sheets from the pad, enter the first caption in the open space, and enter the second caption in a shaded space that matches the open space on the second sheet.
THREE CROSS-REFERENCES
For three cross-references, select the first three sheets from the pad and enter the captions in all three spaces provided.
IDENTIFICATION OF RECORD
In Box 2a, enter the date of the document; the date of the document also serves as the filing date of the form.
In Boxes 2b and 2c, organizational titles or location may be used to identify whom a letter is to or from.
FILED
In Box 3, enter the name, number, or subject under which the document itself is filed.
SPECIAL USE OF CROSS-REFERENCE SHEETS
A single Cross-Reference sheet can be used as a Continuity Reference when correspondence is removed from a cutoff file and brought forward and attached to correspondence in a current file.
When correspondence is removed from a cutoff file and brought forward and attached to correspondence in a current file, use the "Cross-Reference" space to enter the subject from which the document was removed.
When correspondence is removed from a cutoff file and brought forward and attached to correspondence in a current file, identify the document as illustrated on the form.
When correspondence is removed from a cutoff file and brought forward and attached to correspondence in a current file, use the "Filed" space to describe the letter to which this document is being brought forward, including the subject and date. Also show the "To" and "From" of the latest letter if it is different from the letter being brought forward.
Sources
[1] DD Form 2861 (JUN 2003), Cross-Reference, Washington Headquarters Services (the Official source link on this page).

