You can use the W-Item command by equipping a W-Item materia. W-Item lets you use 2 items in a single turn.
Apart from that, the W-Item command can also be used for the W-Item cheat. The W-Item cheat is an easy way of creating unlimited quantities of certain items in Final Fantasy VII. The impression I get is that it's actually not a deliberate cheat built into the game, but more of a software glitch.
First of all you need to know where to find the W-Item materia. In the return to Midgar at the end of Disc 2, when you're in the Subway, head down and keep going along the train tracks. When your team tell you you're going the wrong way, just ignore them. Eventually you'll reach the end where you will find a Save Point and the materia.
Simply equip the W-Item materia and get into a battle. In the battle, select an item using W-Item and then select an ally or enemy to use the item on. Then select a second item (can be the same as the first item, or a different item), but this time don't use it - just select it and then press back. You will see you now have one extra of the first item selected. You can create up to 99 of the item by pressing select then back over and over (it can still be called "unlimited items" as you can create up to 99 items as many times as you like).
This tip is particularly useful for building up Megalixirs and Hero Drinks before fighting Emerald or Ruby Weapon, or Elixirs when fighting Magic Pots. Here you can find out why fighting Magic Pots is important.
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