Editing Tasks with the Task Information inspector
Tasks have a lot of details to keep track of. When you select one or more tasks in the task outline, the Task Information inspector shows the task details.
The task Name is the same as the one that appears in the task outline. 
You can change the Type from a normal Task to a Milestone (a zero-length event which represents an important moment in the project's life) or a task Group, which can contain child tasks. 
Effort is how much total work it will take for this task to be complete. Duration is how much time it will take to get that work done. These can differ based on how many resources are assigned to the task, and the efficiency of the assigned resources. The 
icon sets which value should change when you assign more resources. Note that if you enter a number without any units, OmniPlan assumes that you mean the smallest enabled unit in the Project Formatting inspector. 
The task Cost is how much money the task takes to complete. This is just the cost of the task itself, distinct from the per-use or per-hour costs of resources assigned to the task. 
The Schedule method sets when in the schedule OmniPlan should try to place a task: as soon as possible, on a certain date, or whenever resources become available. 
The Actual start and end dates are when the task really happened during the execution of the project. These can be locked 
against accidental editing, changes from dependencies, and leveling. 
Constraints indicate the earliest a task may begin and the latest it may end. 
The Baseline start and end dates indicate when the task is expected to start and end, according to the original plan. These appear once you use the Set Baseline command on a project. 
The Variance shows how much the actual dates differ from the baseline dates. This is calculated automatically; you don't have to enter it yourself. 
Reserve is an extra buffer of time added to task groups, in case the tasks in the group take longer than expected. Note that there is a difference between no reserve (the checkbox is deselected) and 0 reserve (the checkbox is selected, but the field says 0 because all of the reserve time has been used up). 
The amount Completed is a percentage of how far along the task is. This is also displayed as shading on the task bar in the Gantt chart. 
Editable date fields have this calendar icon; click it to summon a mini calendar window you can use to choose a date and time.