Looking at a recent post on the FFXIV subreddit, it seems plenty of people are making the assumption that there is an additional 50 slots for key items and that these should be used for normal items. I don't believe this is actually true, and that these spaces (visible only when using the expanded inventory UI) aren't actually usable but rather exist for the sake of UI consistency.

If this is the case, and those spaces are just graphical, can they be made to look different to the 25 slots that are used by key items? It would go a way to helping players understand the way that the inventory works. It's also a little frustrating to explain this kind of thing to every person who has yet to realise this.