MIL-STD-2525A
APP-6A's American equivalent standard is MIL-STD-2525A, Common Warfighting Symbology; the contents are essentially identical, but MIL-STD-2525 has been evolving faster than NATO's APP-6. MIL-STD-2525 was issued in September 1994, MIL-STD-2525A in December 1996, MIL-STD-2525A Change 1 in July 1997, MIL-STD-2525B in January 1999, MIL-STD-2525B Change 1 in July 2005, MIL-STD-2525B Change 2 in March 2007, and MIL-STD-2525C in November 2008. APP-6A is considerably different from APP-6, while the successive versions of MIL-STD-2525 more or less maintain continuity. APP-6B seems to be a subset of MIL-STD-2525C.
Read more about this topic: NATO Military Symbols For Land Based Systems