How do units choose which enemy to target in SC2?
Solution 1:
In addition to Tzenes' answer, there are a couple things to note:
- Your units will preferentially attack enemy units that are attacking them. If there are two enemy units in range and only one is attacking, the attacking one will be targeted.
- Your units with instant attacks will not overkill. This has been most frequently noted with tanks. If you have a zergling that wanders in front of five tanks, only one will target him. Units with non-instant attacks, like missiles, can still overkill.
- As illustrated by Tzenes' ATP/OTP tables, the AI will always pick other units in range over enemy workers.
(Additional discussion here)
Solution 2:
So attack priority is based on two factors: ATP and OTP. ATP is used by default, with OTP used as a back up if other units are close. Higher # = Higher priority. During beta units with an OTP of 0 were calculated the same as other OTP numbers. This changed in one of the patches to have the OTP of 0 mean use atp value.
This list can also be found on Liquidpedia
Unit ATP OTP
Protoss
Archon 20 0
Carrier 20 0
Colossus 20 0
Dark Templar 20 0
High Templar 20 0
Immortal 20 0
Intercepter 20 0
Mothership 20 0
Observer 20 0
Phoenix 20 0
Probe 20 10
Sentry 20 0
Stalker 20 0
Void Ray 20 0
Warp Prism 20 0 (identical for both modes)
Zealot 20 0
Force Field 0
Forge 11
Fleet Beacon 11
Gateway 11
Cybernetics 11
Dark Shrine 11
Nexus 11
Photon Cannon 20
Pylon 11
Robotics Bay 11
Robotics Fac 11
Stargate 11
Archives 11
Twilight 11
Warpgate 11
Terran
Auto-Turret 20 0
Banshee 20 0
Battlecruiser 20 0
Ghost 20 0
Hellion 20 0
Marauder 20 0
Marine 20 0
Medivac 20 0
MULE 20 10
Point Defense 20 0
Raven 20 0
Reaper 20 0
SCV 20 10
Siege Tank 20 0 (both modes)
Thor 20 0
Viking 20 0 (both modes)
Armory 11
Barracks 11
Bunker 19
Command Center 11
Engineering 11
Factory 11
Fusion Core 11
GhostAcademy 11
MissileTurret 19
OC 11
Planetary 20
Reactor 11
Starport 11
Supply Depot 11
Tech Lab 11
Note: Flying does not affect priority
Zerg
Baneling 20 0
Brood Lord 20 0
BL Cacoon 10 0
Broodling 20 0
Changeling 20 0
Corruptor 20 0
Creep Tumor 11 0
Creep Tumor 19 0 (Built)
Drone 20 10
Hydralisk 20 0
Infested 20 0
Infestor 20 0
Larvae 10 0
Mutalisk 20 0
Nydus Worm 11 0
Overlord 20 0
Overseer 20 0
Overseer 10 0 (Cocoon)
Queen 20 0
Roach 20 0
Ultralisk 20 0
Zergling 20 0
Baneling Nest 11
Evolution 11
Extractor 11
Greater Spire 11
Hatchery 11
Hive 11
Hydralisk Den 11
Infestation Pit 11
Lair 11
Nydus Network 11
Roach Warren 11
Spawning Pool 11
Spine Crawler 20
Spine Crawler 19 (Uprooted)
Spire 11
Spore Crawler 19
Spore Crawler 19 (Uprooted)
Ultralisk Cavern11
note: Burrowing does not affect priority