You can add an Action object to the scene in several ways:
by selecting a System object and in that object selecting an action from the drop-down menus in the Actions section; this will add an Action object to the scene, but will not link it to any Question object;
by doing the same thing from the Actions folder, which is a child of the System object;
by clicking the Add Action button in a Question object; this will add an Action object to the scene and will automatically link it to that Question object;
by selecting the Action menu entry from the Utilities sub-menu of the X-Particles menu in Cinema 4D; this will add an Action object to the scene but you will have to link it to a Question or other object yourself, if required;
by clicking an Add Action button, which is present in several modifiers and other objects.
When you have created an Action object you can set various parameters in it.
One action - the Editor Display Only action - is always available, even if you select another action type.
With this, you can alter the particle display in the editor, which can be helpful to see which particles are being affected by the action and when they are affected (you can see when an action occurs, by changing the particle color, for example).
You can change the action at any time, by first choosing the general type of action from the Action Type menu and then choosing the desired action from the secondary menu below.
Note: You can turn off an action at any time, by disabling it in the Object Manager or by disabling any Question objects or modifiers which use the action.