By creating authority levels, this ensures that each purchase order is authorised through the correct channels, consistently. In a small company create on authority level, ie. “The Owner”.

In an organisation with multiple owners, create “A signatory” and “B signatory” like you do with banking.
In a medium sized organisation, perhaps you have a supervisor that authorises every PO, so create a “Supervisor” level and then the next upper level, if required.
Go on and build your hierarchy level as it currently exists in your organisation