Can hold Main Hand, One-Hand or Two-Hand weapon. If Two-Hand weapon is equipped, Off-hand slot must be empty unless the character is a Warrior with the Titan's Grip talent.
Can hold Off Hand or One-Hand weapon (Dual Wielding classes Only) as well as Shield (Warrior, Paladin and Shaman Only) or Held In Off-hand item. If Two-Hand weapon is equipped in Main hand slot, this slot must be empty unless the character is a Warrior with the Titan's Grip talent.
When creating macros based on "click to use" items (especially trinkets), the common practice is to enter the actual item name. However Using the numeric designations for an equipment slot is a much more effective method as it eliminates the need to manually modify your macro when you upgrade a particular item.
These Slot designations are particularly handy for Engineers whose Tinker enhancements and Engineering only items add and/or posses an "on use" ability for items in the Back, Belt, Foot and Head slot that would not normally posses such capability for Non-Engineers.
Here are is a complete list of each equipment slot and its numeric designation for macro purposes
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