Military map symbol of a neutral unit or formation. 
This symbol employs the Assault modifier from the US standard in ADP 1-02. The modifier was intended to be used with the aviation icon to indicate air assault lift helicopters. It is instead used with the armoured icon to indicate armoured assault personnel carriers. Symbols such as this would be used for armoured personnel carrier units, such as Canadian Kangaroo units of the Second World War or modern day Assault Amphibian Battalions of the USMC. 

The original APP-6 standard depicted all units within rectangles and had no indicator for neutral forces. Use of the colour green and a square frame were introduced in APP-6A to indicate entities of neutral allegiance.
Own work
CdnMCG