You have a wide range of options when working with purchasing documents. You can decide to go in for very restrictive, detailed planning for a certain project, or give the service provider considerable discretion in performing the required services and concentrate primarily on budgetary control.

Various mixed approaches are also possible. Which one you actually use can be decided from case to case. The following is a list of some of your options:

• You can work only with planned services in the purchase order - and not specify any limits for unplanned services.
  At the time the performance of the services is recorded, only the precise services, quantities and prices set out in the specifications may be entered.
• You can work with planned services and additionally set a value limit to cover unforeseen circumstances. This allows other services to be performed in addition to those specified in detail.
• You can work exclusively with unplanned services. In this case, you enter no specifications, quantities, or prices in the PO. Instead, you merely set a value limit for unplanned services. This allows any services to be performed up to that limit. The services are not actually specified until late in the procurement process - at the time their actual performance is recorded.
• You can work exclusively with unplanned services but restrict the nature of the latter by referencing contracts. In this case, you do not enter any specifications, quantities, or prices in the PO, but set only value limits relating to certain contract items. This ensures that the provider is not able to enter any services at any prices, but only those agreed in the contract.