From Zelda Wiki, the Zelda encyclopedia
The Inventory, otherwise called the Collection or the Item Subscreen, is the location where the storing, showcasing, and equipping of various items, tunics, and swords occurs. The Inventory is, in the Zelda series, a separate subscreen that appears at the press of a button, presenting Link's items, maps, and other miscellany all in one location. Use of the Inventory stops gameplay and time so that the player can select the option, whether it be an item, tunic, shield, etcetera that fits the situation at hand. Because of this, the Inventory can be accessed at any time during the game, except during cutscenes and any screen that is not in-game.
Debut and Split-Screen Inventory
Debuting in The Legend of Zelda, the Inventory not only displays the different types of items Link receives during his journey, but also keeps track of the number of Triforce fragments he has obtained throughout the many dungeons of Hyrule thus far. It is in this title originally that, at the press of a button, the Inventory will replace the game screen, stopping time and gameplay, only to retreat upon another pressing of the same button. Later two-dimensional titles such as A Link to the Past, Link's Awakening, and the Oracle titles carry on this trend, though displaying game-exclusive items and quest specifics.
The multi-faceted inventory is first showcased in Ocarina of Time, with multiple layers: the Map Subscreen lays out the dungeon and overworld maps; the Select Item Subscreen lists items such as Bombs, Arrows, Empty Bottles; the Equipment Subscreen lists Link's clothing, such as swords, shields, and tunics; the Quest Status Subscreen displays Medallions, ocarina songs, Pieces of Heart, Stone of Agony and Gold Skulltula tokens. The multi-faceted inventory is necessary to combat the increasing complexity of the 3D console Zelda titles, with many more items to carry, maps to use, and stats to keep track of. A similar trend is carried into Majora's Mask, The Wind Waker, The Minish Cap, and even Twilight Princess, the latter of which presents a rotating turntable instead of a solid select screen for item selection.
On the Nintendo DS
Phantom Hourglass and Spirit Tracks incorporate the Inventory as a product of the two Nintendo DS screens; instead of flipping through the Inventory like in Ocarina of Time, two screens of the inventory are displayed at once, fully using the capabilities of the two DS screens to the game's advantage. In Spirit Tracks especially, there is a small navigation bar at the bottom of the lower screen that will take the player straight to the Inventory screens, while riding the Spirit Train.