User Attributes are variables that you can use for your own purposes within strategy manager.
They are similar to other attributes (Feature, Tool, Job) in that they are available for use in expressions (see Working with Expressions for more information). Unlike other attributes however you can change the values of user attributes at will, in Data Nodes.
For example, rather than assigning a feature attribute directly to a process modifier (you insert the feature attribute into the expression for the process modifier), you could work indirectly by assigning the feature attribute to a user attribute, then assigning that user attribute to the process modifier. You can now conditionally change the user attribute within the strategy flowchart (you use Data nodes), to influence the output instructions.