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This is where you can equip and/or use items you carry. The tabs on the right are used to change what set of items you can select from your avatar.
For instance if you wish to remove your clothing, click the right clothes tab then click the item on your avatar and drag to the empty window on the left to remove, or go to used for weapon and armor removal. You can equip weapons, armor, tools, etc. by clicking on the tab for them, then drag the item to your avatar. Weight is how heavy your items are, if you carry too much it will slow you down. Count is, of course, how many items you have on you at the time. Balance is your ingame money.