▁red ▁for ▁friendly ▁troops ▁because ▁of ▁the ▁traditional ▁red ▁co ats ▁of ▁British ▁soldiers . ▁However , ▁the ▁system ▁now ▁in ▁use ▁is ▁broad ly ▁based ▁on ▁that ▁de vised ▁by ▁the ▁US ▁Army ▁Corps ▁of ▁Engine ers ▁in ▁ 1 9 1 7 . ▁The ▁inf antry ▁symbol ▁of ▁a ▁salt ire ▁in ▁a ▁rectangle ▁was ▁said ▁to ▁symbol ise ▁the ▁crossed ▁bel ts ▁of ▁an ▁inf antry man , ▁while ▁the ▁single ▁diagonal ▁line ▁for ▁caval ry ▁was ▁said ▁to ▁represent ▁the ▁sab re ▁bel t . ▁With ▁the ▁formation ▁of ▁NAT O ▁in ▁ 1 9 4 9 , ▁the ▁US ▁Army ▁system ▁was ▁standard ized ▁and ▁adapted , ▁with ▁different ▁shapes ▁for ▁friendly ▁( blue ▁rectangle ), ▁host ile ▁( red ▁diam ond ) ▁and ▁unknown ▁( yellow ▁qu at ref o il ) ▁forces . ▁ ▁AP P - 6 A ▁was ▁prom ul g ated ▁in ▁December ▁ 1 9 9 9 . ▁The ▁NAT O ▁standard ization ▁agreement ▁that ▁covers ▁AP P - 6 A ▁is ▁ST AN AG ▁ 2 0 1 9 ▁( ed ition ▁ 4 ), ▁prom ul g ated ▁in ▁December ▁ 2 0 0 0 . ▁AP P - 6 A ▁replaced ▁AP P - 6 ▁( last ▁version , ▁July ▁ 1 9 8 6 ), ▁which ▁had ▁been ▁prom ul g ated ▁in ▁November ▁ 1 9 8 4 ▁( ed ition ▁ 3 ▁of ▁ST AN AG ▁ 2 0 1 9 ▁covered ▁AP P - 6