Value references can be used in macro parameters, sometimes macro body, and even Conditions. However, the actual availability depends on two factors:
For a full list of places where value references can be used, see: see Conditions, Content and Value References – but . But remember it's the reference type and macro which determine the specifics.
If a value reference is requested then the value references are resolved in the following order:
1. Confluence Page Properties macro
2. Comala Metadata which includes workflow parameters with edit=true that have been saved and set-metadata
3. Communardo Metadata
4. Workflow parameters saved at the space level - without edit=true
5. Page references
6. Suppliers (ServiceRocket, Workflows, Metadata)
This order is important when there are conflicts in value reference names.
In your workflow you might need to refer to the author of a piece of content. You won't know this information in advance. To get round around this problem, you can use the
@author@ value reference, which is one of the references that the Workflows app creates and updates automatically. When a macro which that uses the reference is activated, the reference gets replaced with the username of the person who most recently created or edited the content.