Material written by karajorma : http://homepage.ntlworld.com/karajorma/FAQ/fredopen.html#conditionals
What are the new conditionals?
In the retail version of FRED2, all events began with the plain old boring when SEXP. You can do a lot with when. In fact until mid-2004 that was all you had. However, there are some times when when just isn't enough. FS2_Open has added several new conditionals that fill in some of the gaps.
Every-Time is a version of when which continuously repeats itself for the entire duration of the mission. Unlike when, an event using every-time can't ever become true or false. It is always incomplete regardless of however many times it has executed. Every-Time can basically be thought of as being the similar to a repeating event using when with a delay of 0. Due to the fact that events using Every-Time are never marked as true or false, it can be used to solve problems with SEXPs like key-reset in repeating events. On the other hand, you should never chain an event to one with every-time or use the event-true/event-false SEXPs with it as due to the fact it is always incomplete those events will never trigger.
When-Argument / Every-Time-Argument
Probably the two most confusing of the new SEXPs for most people, these two SEXPs are also amongst the most powerful. Both SEXPs allow you to do things that previously would have required a lot of tiresome fiddling with variables. In addition, they also allow you to do things in one event that previously could have required hundreds.
The event works quite simply. When the mission is running, FS2 looks at the list of ships this event applies to. Wherever it sees the word argument, it sticks in the name of the wing at the top of the list. If the condition is true (i.e Alpha 1 is less than 2000m of Cancer wing) then the event triggers and whatever you've replaced do-nothing with will happen. If not FS2 goes back this time substituting the second name on the list wherever it sees argument. FS2 continues down the list until it reaches the end or finds something that triggers the event.
It's worth noting that there is no requirement for the argument to be a ship or wing name. It can be anything, subsystem name, message name, event name, whatever you want.