The key elements are the essentials you find on your trip. They include items such as Vs. Seeker, Poffin Case, Explorer's Kit and Bike. Each of these items can be registered so you can use them without having to log into your inventory. In previous games, you could only register one object. That number has now been increased to four. Here's how to register and use these items in Pokémon Brilliant Diamond and Brilliant Pearl.
To begin the item registration process, you need to access your inventory by pressing X and scrolling to the key items tab. Once there, select the item you want to register by pressing the A button. After selecting the item, you will see an option to register it. Select record and another menu will appear so you can choose which direction it should be recorded in.
Now that the item has been registered, you can access the registered item menu by pressing the + button. The same menu that appears when you register an item will appear. Press the direction of the object you want to use with the D-Pad or left stick. For items like the bike, you can go back to this menu and select the item again to delete it.