Officially known as a Distinctive Unit Insignia or DUI for short, the unit crest of the 21st Support Brigade was originally approved for the 21st Direct Support Group on 5 June 1969. It was redesignated for the 21st Support Group on 20 February 1991 with revised symbolism and description before being given its final designation for the 21st Support Brigade on 7 November 1997. In October 1999, 21st Support Brigade was merged with 3rd Brigade, 91st Division, with the new unit being designated as HQ, 3rd Brigade, 91st Division.
The insignia’s “crest,” a red-and-blue taeguk taken from the flag of the Republic of Korea to commemorate the original unit’s activation and service in South Korea, is supported by a gold Korean “A” Frame, a device create to support heavy supply loads. It signifies the direct support mission of both the original unit and its successors, including the 21st Support Brigade. The taeguk’s two parts (red and blue, often referred to as yin and yang symbols) and the upright frame are an allusion to the unit’s numerical designation.
“Sustain Supply Maintain” was the motto of the now-inactive Brigade.
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The Distinctive Unit Insignia is worn by all Soldiers (except General Officers) in units that have been authorized to be issued the device. It is worn centered on the shoulder loops of the Army Green Service Uniform (AGSU) and the blue Army Service Uniform (ASU, Enlisted only) with the base of the insignia toward the outside shoulder seam. DUIs are not worn on the Dress variations of either uniform, however.
Full guidance on wear of the DUI is found in DA Pamphlet 670-1,
Section 21-22, "Distinctive unit insignia" and 21–3(d) and (e),
"Beret" and
"Garrison Cap," respectively.