copy from Perp Wiki, still mostly valid but needs to be extended with Goblins, Bosses (who have their own category) and more detailed stats
The NPCs are divided into 5 main ranks for each faction with an additional "training" tier (T0) on the alpha islands. These ranks reflect the potential danger of engaging a robot and the possible loot that can be obtained from destroying it. The higher the rank, the higher the risk and reward.
But since NPCs differ in several aspects, it is not always a given that a T3 NPC robot will always be easier than a T4 NPC robot.
The following table gives an overview of the elite NPC prefixes for each faction.
Observers are more powerful than any other NPCs. On Alpha islands you will encounter grand observers which reflect T4 equipped robots with maximized extensions. On Beta and Gamma islands superior observer can be found. These robots have a higher level of skill than T4 equipped robots with maximized extensions.
They tend to patrol a significant part of the island (frequently in pairs), so beware when exploring or mining.
Powerful (but quite rare) NPC robots that vary in strength and provide generous loot.
They're mostly seen in rarely visited areas on islands (at this moment only Imperial Weaver tends to be near a frequently used Bornham Route).
The most powerful bosses (Pitboss Alpha, Superior Invader) tend to walk around the island (and in case of Superior Invader, spawn in different Beta 1 Island, unlike Pitboss Alpha, which always spawns in Hershfield Alpha island).
NPCs robots use their racial weapons and are weak to their opposite faction's weapons. The table below details the damage they deal and which type of damage they are weak to.