In Content Server, attributes exist inside Categories. Categories can have a single attribute, but often have more than one. A Category defines the sequence, layout, type, and default values of the attributes that it contains. Applying a Category to an item means assigning the Category’s attributes to it. It typically means assigning values to the Category attributes too. You can apply multiple Categories to an item.
Example 1-1: Applying Categories to an item
In your deployment of Content Server, you assign the Proposal Category to a document that details an opportunity to create and sell a new kind of valve. The Proposal Category has three attributes: Publication Date, Product Type, and Total Proposal Value. You set Publication Date to today’s date, for Product Type, you enter Custom Valve, and for Total Proposal Value, you enter $50,000.00.
You can apply Categories when you add, edit, or move items. The Add page for most item types includes a Categories section that you can use to apply a Category to an item when you create it. Most items also have a Categories tab on their Properties page where (if you have sufficient permissions) you can change the Category attributes applied to the item or remove the Category from it altogether. You can apply any Category that you have permission to use by browsing to it, selecting it and, optionally, setting its attribute values. Typically, Categories are stored in the Content Server Categories volume, which is a special location that can be accessed by all Content Server users.
If you add an item to a container that has a Category with required attributes associated with it, you must enter values for the required attributes when you add the item. A Category with required attributes is indicated by the Required icon 
.
Categories are versionable items, which means that Content Server can maintain multiple versions of a Category. If you modify a Category, Content Server creates a new version of the Category when you save your changes. If you apply the Category to an item that does not yet have it, Content Server applies the current version of the Category, but Content Server items that already have the Category retain their version of the Category. This allows you to change the attributes in a Category at any time without invalidating the attribute data of items that already have the Category.
Although a new Category version is not automatically applied to items that already have a given Category, you can choose to upgrade an item's Category to the newest version of the Category. If a Category has been modified, the Upgradable icon 
appears on the Categories Properties tab of every item that has that Category. It indicates that you can upgrade the item's Category.