This is most common method. It gives most control over actions of unit, but it is more prone to misclicking, and may not be possible if infected is approaching a tall building from north, in which case unit will attack building instead. It can also spread damage if used against crowds.
In this method the unit will just attack the closest enemy. It doesn't work for units with looping attacks, though.
Units and buildings in game files have multiple properties responsible for their attack speed:
TimeLoad - time needed for unit to raise their weapon. For example, Thanatos takes 1.5 seconds to sit down and take out rocket launcher.
TimeAction - time needed for unit to go through their full attack cycle animation.
TimePreAction - The unit will wait this time before executing the action (ms).
TimePostAction - The unit will wait this time after executing the action (ms). Only used by Venom
TimeLoopAction - If this parameter has value the action is continuous and this is the loop time (Lucifer and Mutants).
TimeUnload - time needed for unit to lower their weapon before they can move again.
Attack rate shown in interface doesn't exist in game files, but is calculated from these properties. It represents amount of attacks per second. It's equal to 1/(TimePreAction + TimeAction + TimePostAction), or 1/TimeLoopAction.
Animation canceling happens if player manually interrupts animation during TimeAction phase right after unit releases a projectile. It's most useful if actual attack happens early in animation, and unit doesn't have long TimePreAction. For example, Ranger's animation takes 1000 miliseconds; she releases tendon around 300 miliseconds, after which reaches into quiver for an arrow and draws a bow. The part after those 300 miliseconds can be skipped. For Soldier and Titan the recoil part of animation can be skipped.
Units with TimeLoopAction can be forced to attack every frame, as long as player clicks fast enough.
AC can be "patched" by setting TimeAction to be short and either increasing TimeLoad or TimePreAction.