- For how to define an item, see Defining an item.
- For the effects that items have, see Item effects.
- For giving, taking and counting items in the Bag, see Manipulating items.
- For putting an item in a map for the player to pick up, see Item balls.
- For information about Poké Marts, see Poké Mart.
An item in the code is nothing more than an ID symbol (e.g.
:POTION). There is no difference between any two of the same item.
Mail can display custom messages. A mail's message (and related information) is saved as a property of the Pokémon holding the mail. The mail item and its message can be stored in the PC's Mailbox instead of the Bag. The message cannot exist otherwise.
An important item is one which the player should be able to obtain only one of throughout the game, and which they will not be able to get rid of (i.e. by tossing it, selling it or using it up). The quantity of such an item will not be shown in the Bag.
Key Items, TMs and HMs are classed as important items, by defining them as such. Note that TRs are single-use versions of TMs, and are how TMs used to work prior to Gen 5. The difference between TMs and HMs is that moves taught by HMs cannot be forgotten except via the Move Deleter.
Since important items cannot be sold, their price is (usually) irrelevant. If the price is non-zero, however, it can be sold to the player by a Poké Mart. The reason the player cannot sell an important item is because it is classified as an important item, not because its price is 0 (although items with a price of 0 cannot be sold either).
It is worth noting that, although an important item's quantity is not shown in the Bag, it is possible to have multiples of an important item at once. Unless you have a specific purpose for doing so, though, you should check whether the player already has the important item before attempting to give it to them again.