The critical path is the chain of activities with no room to slip: delay any one of them and the project finishes late. Try it below, then run it on your own programme.
On a schedule with constraints or multiple calendars, "float = 0" and "driving the finish" are different sets. Aimavera follows the engine's own driving edges — the chain that answers "what makes the project finish when it does".
Total float is how far an activity can slip before it starts costing the finish date. In the demo above, lengthening Fabrication spends the design chain's float — then the path flips and every further day is a day late.
In the Studio there is no Schedule button. Change a duration, add a link, move the data date — the whole network reschedules as you type, and the red path moves with it.
Open your XER or MS Project file and see your own critical path — free, in the browser, nothing uploaded.