By default an Action runs by itself at a certain Runtype (naturally provided that a user can see/is targeted by an Action and that Conditions are matched, if any). 

If an Action is set to "Run on reference only" it will only run when referenced or being depended on by another Action.


An Action can be referenced by another Action

NOTE: Not all Actions provide sense-making results for referencing; where useful such examples are provided in the detailed documentation of the respective Action.