Weapons.dat - Target Flags

0x001 Air

0x002 Ground

0x004 Mechanical

0x008 Organic

0x010 non-Building

0x020 non-Robotic

0x040 Terrain

0x080 Organic or Mechanical

0x100 Own

This is actually better thought of "which units can this weapon deal damage to". Eg if you give a ground weapon the air + ground target as well as splash, then when it attacks a ground unit, any air units in the vicinity will also take splash damage. If you also set it to have Mechanical targeting, then the weapon will be able to attack all types of ground units, but only deal damage to ones with the mechanical flag.