FX5 box says filtration volume 20L so I assume it means how much water it holds. It will also tale 5.9L of media.
The time my wet/dry stayed boxed had nothing to do with why it broke 6 months after use. It just meant we could not claim a replacement under warranty. I guess I will have the same problem with this filter as I have no proof I bought it a long time before it was actually used.