The call out concept will replace a Reserve/Standby activity with a Call out activity if you assign flights that overlap the Reserve activity.
Create a Call out activity of type Reserve or Standby.
Reserve or Standby
When you drag the flights down on the reference activity, the original activity will be replaced with the adjusted Call Out activity.
Note: The Type replace the same Type. Standby replaces a Standby, Reserve replaces Reseerve.