About UK Milliradians
The UK milliradian is one thousandth of a radian, or 0.0573°, so 6,283 of them fit in a circle. At 1 meter it spans 1mm.
An object that fills 1 milliradian of your view is 1 meter wide at 1km, or 1mm wide at 1 meter. Divide the width by the angle to get the range – a 1.8m person covering 3 milliradians is 600m away.
Degrees don't give you that shortcut: at 1km one degree is about 17.45m wide, which is nobody's idea of a round number.
About USSR milliradians
The USSR mil was the angle unit of Soviet artillery. It is a rounded version of the milliradian, within a few percent of one thousandth of a radian.
Soviet artillery used the mil in its sights and for estimating range: a target of known width that covers a measured number of mils gives the distance.
Like the other mils it is a military convention rather than an SI unit – each army chose a convenient number of divisions for the circle, so check which mil a figure refers to.
There are 6,000 USSR milliradians to a full circle.