As a former old school TL I can tell you what happens.
Basically EXF was used a lot "back in the day" before "outs" was even a dream in a programmers eye.
Let's say you have one location in the store of PS Vita's. It has a capcity of 5 with one facing. Let's assume it was empty and was shot with OUTS, thus resetting the accumulator. Let's assume it generated a batch of 5 and it was properly pulled and stocked.
Normally (assuming none of them are stolen) the system will automatically generate pulls to replenish them based off a number of 5. So, if someone comes in the store and buys all 5 one day the system will generated more to be pulled.
Now, let's assume your electronics TL (or your handy PS rep, i.e. ME!) decides one facing just isn't cutting it. They decide to put another facing with a capacity of 4. So, now there are 9 PS Vita's on the sales floor. Your electronics TL doesn't need 5, so they decide to shoot an EXF of 4.
Now the system "knows" that there are 9 total PS Vita's on the floor. It doesn't know where the other 4 are, but it knows they are somewhere.... just not on a planogram.
Now the system will pull as if there are 9 that need to be filled on the floor instead of 5.
Normally, this is fine. It only causes a problem if your electronics TL does something crazy.... like creates the new flex location of 4 PS Vita's in the Ipod case. Thus, when your logistics team is trying to stock them they say "WTF? There is only a capacity of 5 and I pulled too many!" because they have no idea there is a flex location in the Ipod case.
Also, it causes a problem once the flex location is eliminated. Until the item is shot with outs again, the system has no idea the flex location no longer exists.....
Other than those issues, EXF is useful.