Military map symbol for a friendly 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 blue frame and icon were defined in APP-6 to indicate friendly. In APP-6A, the friendly frame and icon could be produced in blue (typical for hand drawn or on when used over a bright background in electronic display) or cyan (for use over darker backgrounds in electronic display). In APP-6, all units were represented by a rectangle with dimensions that varied from 1.5 by 1 to 1.7 by 1. The 1.5 by 1.0 frame was stated as the standard dimension for a friendly unit in APP-6A.
Own work
CdnMCG