EvalSubUnitCount = <Operator>;<Value>
Once you do a selection of sub-units (aka elements) with SelectSubUnit(s) you can want to do a test versus the number selected. This is where the command kick in. It is much more powerful than the old CountUnits, which is counting at the unit level and have less selection possibilities.
The valid operators, are, as usual: <, >, <=, >=, =, DICE
For example
EvalSubUnitCount = <=;10
Is: 'do I have selected at most 10 elements with SelectSubUnits'. The 'March to Richmond' event can be vastly improved with that, counting only line regiments and cavalries for example.