Declarative business rules amount to applying annotations on the appropriate methods. But you should note that Naked Objects does not (currently) support annotations on fields, so it's necessary to put the annotation on the getter for the property.

For example, to indicate that a property is disabled (read-only), we can write:

class Claim ... {
    String status
    ...

    @Disabled
    String getStatus() { status }
}

Other annotations are used as hints for the user interface. For example the @MemberOrder is used to specify the order in which properties and collections appear in the UI:

class Claim ... {
    String status
    ...

    @MemberOrder("3")
    String getStatus() { status }
}

Another annotation is @Named, which commonly appears on action parameters if using built-in value types. For example, the Claim's addItem() action looks like:

class Claim ... {

    void addItem(
            @Named("Days since")  int days,
            @Named("Amount")      double amount, 
            @Named("Description") String description) { 
        ClaimItem claimItem = newTransientInstance(ClaimItem.class)
        Date date = new Date()
        date = date.add(0, 0, days)
        claimItem.dateIncurred = date
        claimItem.description = description
        claimItem.amount = new Money(amount, "USD")
        persist(claimItem)
        addToItems(claimItem)
    }
    ...
}

The full list of annotations can be found in the Naked Objects applib, and are of course documented in the Naked Objects documentation.