REactions
Reactions are effects that are a response to a condition such as being attacked or being looted via the Scavenge skill. Defenses and Aftereffects are a type of Reaction (and some Defenses are Aftereffects). Backlash attacks (e.g. Project Halo, Venomous Blood) are Reactions to being looted or suffering a Death Blow.
The only rules regarding Reactions are as follows:
You may only use one Reaction per situation. For example: if Marco is attacked by Olivious, Marco can invoke Dodge to defend it or use his Furious Rebuke skill to avenge the slight. He cannot use both.
Reactions and Aftereffects are very similar except for one difference: Aftereffects cannot follow up after other Aftereffects or Reactions but an Aftereffect can follow Reactions. To sum up:
Reaction —> Aftereffect
Aftereffect —X Aftereffect
Aftereffect -X Reaction