Current sensors names as of 2017-02-21 are: "radar", "infrared", "xray" and "spectral".

Spectral droids use default behavior: Attack only what they can see and reach. They are using standard visual modules.
X-ray droids can see though walls, but cannot see invisible tux.
Infrared droids can see invisible tux, but cannot see though walls.
Radar droids uses GPS and can see invisible tux even behind a wall.

For future was also planned: "subsonic"

Subsonic droids detects soundwaves/vibrations and therefore see tux as long that Tux is doing any action which produces sounds (heartbeat and breathing are desconsidered)

Special Forces

Special forces is how are called droids from the Level Editor. They are non-random and they're not script-generated either. Special forces droids are saved on data/storyline/actX/ReturnOfTux.droids

Most of fields are created and filled by the level editor with some exceptions, usually optional, which are explained here.

UseSensor field

Droids aren't forced to use the default sensor from their class. This is specially useful when creating boss-like droids, like the ones on Master Arena. UseSensor is an optional field which requires a string.

Example: UseSensor="spectral"

Marker field

A marker identifies a group of droids/NPCs. They're mostly used by quests where your mission is to kill a group of droids.

Structure: Marker = Group(1+) + Level(3 digits); Example 1037 for group 1 on lvl 37
Groups that need to span several levels belong to level 999

Please note that 9999 used to be a special marker. Please do not modify.