You access the Item Detail window when creating a new Item or editing an existing Item through the Item window.
The following options are available for Item:

Information Name The name of the Item
Information Barcode The barcode of the Item
Information Brand The brand of the item (Brand and Main Supplier must be associated, see Brand Supplier Association article)
Information Main Supplier The main supplier of the item (Brand and Main Supplier must be associated, see Brand Supplier Association article)
Information Class The Class tree on the top right of Item Detail window determines the Class of the Item (see Class article)
Information Stock Section  The bottom part of Item Detail window is the Stock Section, see the explanation at the bottom of this article
Advanced Image URL The image URL of the Item (See Dropbox Public URL article)
Advanced Base Cost See Item Cost Price article to understand how Gazelle determines an Item's cost price
Advanced Calculated Cost See Item Cost Price article to understand how Gazelle determines an Item's cost price
Advanced Item Markup % Set this to be 0, this is not currently used by the system
Advanced Mass / Volume Mass / Volume of the Item
Advanced WWW If this is ticked, the item would appear in Gazelle Online Store
Advanced Supplier If this is ticked, the item is a Service Item (see Service Item article)
Advanced Supplier Service If this is ticked, the item is a Supplier Service Item (see Supplier Service Item article)
Advanced Taxable If this is NOT ticked, the item is non-taxable in Sale transactions 
Advanced Track Profit Gazelle includes functionalities to track profit, untick this to disable this behaviour for the Item. For instance, when the Item is a self produced Item or a Service Item 
Variation   The Variation tab is used when the Item is a member of Variation Group (see Variation Group article)

Stock Tab

The Stock Tab allows you to define stock for stock items (those items that you keep track of their inventories). On the upper section is the Stock table showing the stocks/inventories maintained for this Item in the different Branches that you have. To delete a stock entry, right click on the stock row in the Stock Table and click Delete Stock. To create a new stock entry, fill the stock information on the right of the Stock Table and click Save Stock

Branch The branch that the stock belongs to
Stock Qty The stock quantity. You can't edit the stock quantity manually since it is being maintained by Gazelle. On creating an item, make sure Stock Qty is set to 0
Low The low quantity warning. When you are creating a Sale and the stock falls below this quantity, a warning will be shown to the Staff
Price The price of this Item in the Branch (this will override the Price option)
Override Branch Tax Code Tick this and select a Tax Code if you want this item to have different tax from the branch's tax

Also see Stock Item and Non-Stock Item article