It is the path of 0 Total Float Through the project
Critical path project management (CPM) is a technique used to complete projects on time by focusing on key tasks. One path through all the inter-connected tasks is the fastest avenue to take when completing any project. By focusing on the tasks that make up the critical path, the project manager maximizes the chances of completing the project on time.
No Project Deadline
By setting a deadline on the project (a Must Finish By date in Primavera P6), it results in the appearance of negative Total Float if you are late, and positive float if you are early.
Constraints affect an activity’s Total Float value. Constrained activities can show negative Total Float and can drive negative float to their successors or predecessors. Applying constraints throughout a project will affect what activities are Critical. And so, again the path of 0-total float through the project eludes you.
A project with actuals may not show a path of 0-float either. The actuals and the order of execution can affect the activities’ Total Float as well.