Large ships with big weapons have a huge advantage against lots of smaller ships with smaller weapons, even with equivalent point size. This is designed to try and balance that a bit. It also leans into the trope of big guns being unable to track small fast targets, even if speed of light weapons shouldn't have this limitation.

The bigger the gun, the more difficulty it has tracking small targets. If a target has a MASS smaller than that listed for the weapon class, then it cannot be targeted by that weapon as long as it has a thrust of at least 4.

Weapons without a weapon class (such as scatterguns or PDS) are considered class 1.

A target merely needs to be capable of thrust 4 to be able to take advantage of this rule, it doesn't have to actually spend 4 thrust that turn. If the drive has been damaged, use the damaged drive value.

To allow smaller, often civilian, ships to have some defensive capability without needing to make use of full military fire control and beam weapon systems, PDS can be used against ships in a limited role. It also allows a heavily anti-fighter ship to have some anti-ship capability.

PDS can be used offensively against ships within 6“, as long as they haven't been used for any other purpose that turn. A PDS goes 1pt of damage on a roll of 5 or 6. Against targets with one level of shield, they do 1pt on a 6. They are unable to harm targets with two or more levels of shields.

A PDS does not need to use a fire control when used in this way. They also cannot make use of ADS in any way.