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 (critical path: The series of tasks that must be completed on schedule for a project to finish on schedule. Each task on the critical path is a critical task.). 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 (Yes/No field: A type of field with content set to either Yes or No; also known as a Boolean field or a True/False field. For example, the Recurring field indicates whether the task is a recurring task.)
Critical (task field)
Entry Type Calculated (calculated field: A field in which the value is determined by Project based on information in other fields. Project may recalculate the value automatically or when you specify, depending on your choice of calculation settings.)
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 check box 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)