About Sign
A sign is 30 degrees, one twelfth of a circle, or π/6 (0.5236) radians. It comes from the 12 signs of the zodiac, each covering 30° of the ecliptic, and the word is from the Latin signum, a mark.
About NATO milliradians
NATO milliradians (NATO mils) are a unit of angular measurement commonly used in military and artillery applications. They are derived from the radian, which is the standard unit for measuring angles in the International System of Units (SI). A radian is defined as the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle.
One NATO mil is 1/6,400th of a circle, or 0.05625°, so a degree is about 17.78 NATO mils.
A NATO mil is close enough to a true milliradian (6,283 of those make a circle) that gunners treat it as 1 meter of width at 1,000 meters of range. A circle of 6,400 also divides neatly into quarters, eighths and sixteenths.