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There are 4 different types of approvals:

Product

Is enforced when you have Company → New products must be approved checkbox selected. This type of approval is useful if a requestor wants to introduce a new product or a service that hasn't been purchased before and the company policy mandates that the new products must get reviewed first.

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So, if our user would choose IT & Legal review as a product approval type then the purchase request would have to be reviewed by PO IT Reviewr and PO Legal Reviewer. Once both of them has given their Yes vote, the product will be marked as approved in the products list and another requestor would be able to choose it from autosuggest Description box and no product approvement will be required anymore.

Budget

Budget based approval is basically money-pot approval - you're asking the Approver of the budget to allow you to spend money from that budget. The budget approver is a single person in Approver role who is assigned to specific budget(s) as approver on budget configuration screen. As a purchase request is always connected with exactly one department, we know which budgets are related to that department and can automatically add required approvers to the list of approvals. You, as purchase order requestor, might be required to select an appropriate budget when entering a new order line if case there's more than one budget associated with the department for which you're creating this purchase request.

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You can read more about department-based approvals here

Tier approvals

Finally, we have one more approval type - and that is based on the Gross value of a purchase request. You can configure your own tiers under Company → Company Approval Tiers. Consider the following example of tiers configuration:

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User with role FINANCE can ultimately approve the PO or make any modifications in the approved PO. No matter if other have voted (or how), the vote of the FINANCE user will be ultimate and immediately transit the PO either to Approved or Rejected status.

This is implemented with the idea in mind that sometimes it is necessary to speed-up the approvement process (people might change the job, forget to do the approval or be on vacation). Also, it might be necessary to make corrections even in approved PO. For that reason a FINANCE user has the power to do this things.

As you may probably see here, it is generally not a good idea to have a user with FINANCE role in the list of approvers for the purchase order. This can logically compromise the approval workflow where everybody is expected to give his or her vote for the specific purchase request.