In most factories the production plan lives in two separate places: an order list in the ERP and a spreadsheet on the planner's desk. The list says which order will be delivered when; the spreadsheet holds which machine that order will be made on, in which sequence and with which tooling. The sequence is usually set by talking it through in the morning meeting, is broken by an urgent order that comes in during the day, and by the evening nobody remembers what the first version of the schedule was. This arrangement works in a small operation. As the product range widens, changeovers grow longer and several jobs land on the same machine, it stops working, because the knowledge that sets the sequence is in a single person's head, and when that person goes on leave the factory's planning speed drops visibly.
What is missing here is usually not planning software but which question planning answers. Material requirements planning (MRP) solves the question of 'how much and when do we need it': it calculates the requirement from the bill of materials, counts back over the lead time and produces order proposals. In its classic form, however, it assumes capacity is infinite; it sees nothing wrong in writing three jobs onto the same machine on the same day. Advanced planning and scheduling (APS) comes in at exactly this point and answers a different question: which job, on which machine, in which sequence and at what time. MRP gives the quantity and the date; APS places that date on the machine calendar. The two are not rivals; one uses the other's output as its input.
When APS builds a schedule it takes the constraints into account at the same time: the availability of the machine and the tooling, the set-up time lost in changing from one product to another, the operator and shift pattern, the date the material will enter the warehouse, the windows reserved for maintenance and the priority order of the orders. The critical point is that set-up time is not fixed; going from a light colour to a dark one does not take the same time as going from dark to light, and when two jobs in the same tooling group are run back to back the changeover almost disappears. Getting the sequence right is, in most plants, the source of the capacity gained without buying a new machine. When the schedule is built, two things are obtained at once: a sequence the shop floor can follow, and a due date that sales can give the customer and that is not imaginary.
The condition for setting this work up honestly is to discuss up front how solid the input is. APS cannot produce a better schedule than the routing, standard time and set-up time data it is given; where that data is missing, the system produces plans that look good but do not hold on the shop floor. APS also does not create capacity, does not remove the bottleneck and does not replace the planner; what it does is move the decision from intuition to calculation and make the alternatives comparable. The most common mistake in practice is rebuilding the schedule at every small change and continually sending a new sequence to the floor; the result of this is that the team stops trusting the schedule. That is why the freeze window and the rescheduling rules are defined at the start of the work.