Definitions for "Critical Activity"
an activity that has zero or negative float. This activity must finish on time or the entire project will fall behind schedule.
an activity whose total float is equal to zero
Each activity within the IMG has an attribute assigned to it, indicating whether the task is one where the project team should take particular care when reviewing or changing it. This is because SAP has determined that changing the settings for such critical activities after processing transactions could have serious consequences. Control on these activities during and after an implementation is very important.