Custom fields allow you to add structured information to your resources which you can then use to help you find and manage them more effectively.
For example, you could identify all hosts with their physical location, for instance, or label the cost center and applications running on all of your virtual machines. You simply add a custom field to a server, VM or any other resource in the pool, then give it a value. You can then use custom field values when building search queries.
Custom fields are shared at the pool level. Setting a custom field of any resource in a pool makes it available to all other resources in the pool, on the General tab and in the Custom Fields tab of the resource's Properties dialog box.