The pop culture lexicon often depicts troops from WWII and Vietnam as having a playing card, usually the Ace of Spades, strapped onto their helmet. Although the Vietnam-era "Death Card" is an over exaggeration of troops believing the urban legend that Vietnamese feared the Spade symbol (they didn't) and later as a calling card and anti-peace sign, it remains a symbol for unit identification.
Most specifically within the 101st Airborne Division.
The practice of painting the symbol onto a helmet was created in England just before the Normandy Invasion. The purpose was that when a soldier jumped or glided into Normandy and got separated from a larger portion of the unit, the easily identifiable symbol would easily mark a soldier as being apart of a specific regiment and a small dash at the 12, 3, 6, or 9 o'clock position specified the battalion.
In March 1945, Gen. Eisenhower awarded the 101st Airborne Division with a Presidential Unit citation for defending Bastogne.
Spades were designated for the 506th Infantry Regiment, Hearts for the 502nd, Diamonds for the 501st, and clubs for the 327th Glider Infantry. The 'tic' marks went from 12 o'clock meaning HQ or HQ company, 3 o'clock being 1st battalion, 6 o'clock being 2nd, and 9 o'clock being 3rd battalion.
This French guide was used to identify all units within the 101st and a few from the 82nd Airborne. (Image via Quora)
Today, the symbols are still used as a call back to the 101st Airborne's glory days in WWII.
The regiments are more commonly known by as Brigade Combat Teams and the symbols are each given as Clubs to the 1st BCT, Hearts to the 2nd BCT, Spades to the now deactivated 4th BCT, and Diamonds to the 101st Combat Aviation Brigade along with the 1st Battalion, 501st out of Alaska under the 25th ID. The Rakkasans of the 101st Airborne's 3rd BCT wear a Japanese Torii for their actions in the Pacific and being the only unit to parachute onto Japanese soil at the time.
U.S. Army Photo by Photo by Maj. Kamil Sztalkoper, 4th Brigade Combat Team, 101st Airborne Division