Critical activities have no float (slack). They determine the remaining duration of the turnaround. Most project management software highlight critical path activities in red by default.

Near critical activities are those whose float (slack) is equal to one shift or less. These activities have a high potential of becoming critical.

Activities with float between one and two shifts should be on a "watch list", as they have the potential of becoming near critical.

Due to inaccuracies in scope definition and/or estimating, or logic, it is possible that some near critical activities could be, in effect, critical. Conversely, it is possible that critical path activities, for the same reasons, could in reality have float. This is another very strong reason why the schedule must be updated after every shift, every day, since inaccuracies in logic, scope and estimating will surface during the course of the turnaround.