The Purchase Order field allows the user to select a Purchase Order to be attached to a Billable Task or Transaction.
Each Purchase Order will represent a sum of money to be spent (i.e., billed). It is the Project Manager’s responsibility to monitor the consumption of this money (i.e., revenues). He/she must make sure that all of it is used (i.e., billed), but must also monitor how much work is done so that it does not exceed that which the customer has actually purchased (otherwise, the margin of the project will decrease).