Basically it adds a 50% chance of the event failing, to what ever other conditions are present.
The script engine goes through the Conditions section from top to bottom, and if all statement are Boolean "True", then the event fires successfully and does what is in the Actions section.
A quick explanation of Events.
Nearly always before the event is actually evaluated, there is a statement selecting a faction, and sometimes a region. These are for the script engine to use as a default reference during evaluating the event.
'StartEvent' starts the script engine evaluating the event, and ends with 'EndEvent'.
The parameters are separated into fields by '|' (pipe character). The position of a field within a statement determines the meaning of the field content.
eg: StartEvent = USA_DistantBlockade_FtMonroe|999|0|NULL|NULL|$Ft_Monroe_VA|NULL
USA_DistantBlockade_FtMonroe : Unique Event Name
999 : Occurrences - How often an event may be evaluated.
- 0 = Never (although other events can change this value to turn an event on or off.
- 1 = The event will be evaluated, until it was completed successfully once, or time-frame conditions make it illogical. More to that later.
- 999 = The event will always be evaluated, unless there are time-frame conditions make it illogical.
- From 2 to 998 = Event may fire successfully this many times. This is valid, but very rarely seen.
0 : Type of Event.
- 0 = Quiet Event, the event will not output an event message to the message box nor have a pop-up.
- 1 = Line Event, a 1-line fixed text from the strings database will appear in the message box, if the event was successfully fired.
- 2 = Pop-up Event, not only is a line message generated, but if you click on the line text in the message box, a pop-up will be displayed, generally with an illustration, some extra text (often simply historical flavor), and possibly some accompanying sound or song.
NULL = If a pop-up event, the pop-up event's pop-up text-name. The text itself is found in the strings database.
NULL = If a pop-up event, the pop-up event's graphics.
$Ft_Monroe_VA : If the event fires successfully, AND is at least a Line Event, clicking on the Line Text will position the map to this region.
NULL = If a pop-up event, the pop-up event's sound file.
Nearly all events have MinDate and MaxDate, which determine the time-frame within which the event may successfully fire.
There are very many conditional statements, which may be evaluated. Probability is simply one of them.