Attributes have two types of Classes:
Standard Attributes - behave like Key Words and can be used to profile your count - this section deals with Standard Attributes. 
Numerical Attributes - are used as multiplying scaling factors for quantitative reporting. For more information on Numerical Attribute Categories see Setting Up Numeric Attributes for Reporting
The Attribute tree represents the characteristics and conditions that you want to assign to nodes in other trees in order to selectively group them for analysis. For example, you may have an Attribute Category called Priority for Implementation, with the priority levels (Mandatory, Optional, Later) as its Attributes. You could then assign the priority for implementation to the individual Processes and Data Groups using the Link Nodes function, and use the Filters and Flags to analyse and report the total Function Points assigned determined to be mandatory for this Release.
The more information that is placed within an attribute tree and Linked to other tree nodes, the more detailed and comprehensive your analysis can be.