The Critical fields indicate whether a task or an assignment's task has any room in the schedule to slip, or if a task is on the critical path. The Critical field contains Yes if the task is critical and No if the task is not critical.
There are two categories of Critical fields.
Data Type Yes/No
Critical (task field)
Entry Type Calculated
How Calculated On the Calculation tab of the Options dialog box, you can enter a value in the Tasks are critical if slack is less than or equal to box. The default for this option is 0 days. If the value in the Total Slack field for a task is less than or equal to the value you entered, the task's Critical field is set to Yes. If the value in the Total Slack field for this task is greater than the value you entered, the Critical field is set to No.
Best Uses Add the Critical field to a task sheet when you want to see which tasks are critical and cannot slip without the rest of the schedule slipping as well. You can also filter for this field to view all the critical tasks in the schedule.
Example You're preparing for a status meeting, and you want to review which upcoming tasks are critical. You filter for the Critical field to see only critical tasks.
Remarks You can calculate multiple critical paths. If the Calculate multiple critical paths checkbox on the Calculation tab of the Options dialog box is selected, each independent network of tasks within the project is considered a mini-project for the purposes of late date and critical calculations.
Critical (assignment field)
Entry Type Calculated
How Calculated If the task for this assignment is critical, then this field is set to Yes. By default, the task is critical if there is zero slack. This means that if the task is delayed, successor tasks and the project finish date are delayed.
Best Uses Add the Critical field to the Task Usage or Resource Usage view when you want to view, sort, filter, or group the assignments by critical tasks in your project. This is particularly useful when you're adjusting assignments and you want to make sure that the adjustments you make don't affect the scheduling of critical tasks.
Example You're adjusting work on specific assignments to resolve resource overallocation, but you want to make sure that the changes you make don't affect the project finish date. Add the Critical field to the Resource Usage view so you know which assignments have the least scheduling flexibility.