An order marked as ASAP is scheduled starting from today and works forward until it is completed. This means it will allocate or buy stock first, then allocate resources when the stock is available. This is useful when you want to know when work on an order would finish if you started now. By contrast, an order marked as JIT is scheduled starting from the required completion date, and works backwards until it finds when it should start. In this case, resources are allocated first to find out when the stock is required, and then stock is allocated or bought for that date. If the start date is found to be earlier than today, it means you cannot meet the required completion date.
In addition to these two basic strategies, an order can also be marked as urgent. Normally, the scheduler will make use of work in progress, even if that work is late. When an order is marked as urgent, the scheduler will try to beat late work by using whatever alternative suppliers, materials and resources it’s been given. If it succeeds, it will use the alternative it found instead of waiting for the late work in progress.