Sentinel frigates refuse to target some ships

[Ricca]

If a ship has radar dampening range of, say, 5000, hypothetically, then units with a range of less then 5000, like fighters and bombers are unaffected, since they have to get closer then 5000 units to deal damage.

Missile frigates are affected, since they have a range of 10,000, so they have to close in to 5000 range to hit the hypothetical unit.

That's pretty much how radar dampening works.

[Hearteater]

Short Version: Radar Dampening is the maximum range the unit can be attacked from.  So low numbers are better because enemy ships need to get closer before they can attack.