When policies are created and a user creates expenses, Avianis will first look for a trip policies. If none exists, it will then look for an aircraft policies. Lastly, Avianis will check expenses against the Company Policy. It is advised to set up a Company Policy, at a minimum. This guide covers: 

See related articles:
Finance Policies (Aircraft)
Finance Policies (Trip)
Expense Approval Users

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The Company Policy is created in Finance > Expenses > Expense Policies 

Note: Only one Company Policy can be created and is a general policy for expenses that are not tied to an aircraft or a trip. A company policy MUST exist in order to leverage the expense report approval flow. 

Here is an example of a company policy: 

Receipt Limit: The amount at which a receipt must be provided
Amount Limit: The amount at which the approval process is initiated
Expense Categories: Those selected will become what the crewmember has available to select when creating an expense of that type (in this case, a ‘general’ expense).
Users for Approving Expenses: Set a group, personnel or custom role that can approve another personnel's expense report when this policy applies. When a group is set, only one personnel within the group must approve or reject the report. When one member of the group approves or rejects the report, the report will be cleared from the My Approvals tab of the other group members.

Ref: F-F09