Very often in constraint programming, the efficiency of the decision-making process depends on the appropriateness of the order in which decisions are made. Here, it is very important to make two selections appropriately: to select the resource to make decisions about, and to select the activity to schedule first with respect to this resource.

Scheduling problems are generally such that resources are not equally loaded. Over some periods of time, some resources are more relied upon than others. These resources are often called critical: their limited availability is a factor which prevents the reduction of project cost or duration. It is, in general, very important to schedule critical resources first, in order to optimize the use of these resources without being bound by the schedule of other resources.

A standard way to measure the criticality of a resource consists of comparing the demand for the resource to its availability (supply) over a specific period of time. Here, for example, the period under consideration may run from the soonest of the earliest start times of the activities to order, to the last of the latest end times of these activities. As the capacity of the resource is 1 at all times, the supply over this time interval is the length of the interval. The demand over the interval is the sum of the durations of the activities to execute.

Slack is defined as the difference between the supply and the demand. The most critical resource is the resource with the least slack. Only the activities that are not yet ranked need to be considered in the computation of demand and supply, because once an activity is put in order, there are no more decisions to make about it.