To enable an approval rule, it must be assigned to at least one approvee role (to determine which employees are affected by the rule) and one approver role (so the system knows who the transactions should be sent to for approval).
Like approval rules, a policy rule is added to an approvee role to determine which employees are affected by the policy rule. However, unlike approval rules, you do not need to add a policy rule to an approver role.
Approval rules that are not assigned appear at the bottom of the tree diagram on the Module - Workflow Roles screen.