Scheduling Fundamentals
Longest Path
Longest path is an alternative definition of 'critical' -- the single longest continuous chain of logically-tied activities from start to finish -- which P6 lets a project use instead of the more common zero-float definition.
Definition
Most schedules define 'critical' as any activity with zero total float. P6 also supports a second definition, set at the project level: the longest path, meaning the single longest continuous chain of logically-connected activities running from project start to project finish, regardless of whether every activity on it technically shows zero float. The two definitions usually agree, but not always -- and if a project's critical-path-type setting changes between two schedule updates (deliberately or by accident), the set of activities flagged as 'critical' can shift purely because the yardstick moved, not because anything about the actual work changed. That's a serious problem for any delay analysis spanning multiple updates: comparing critical-path status across two updates that used different definitions means comparing two different questions, not two states of the same schedule, and any conclusion drawn from that comparison is unreliable until the methodology change itself is accounted for.
Example
A project's critical-path setting is changed from 'Total Float = 0' to 'Longest Path' partway through construction. An activity that was never on the zero-float critical path suddenly appears as critical in the next update -- not because it slipped, but because the definition of critical changed underneath it.