Differences Between Rollup and Summary Tasks:
The difference between rollup and summary tasks found in Plan Board would come down to how the tasks completion rates are calculated.
Let's take the scenario of a parent task with a 2 month duration period. If the rollup task with sub and daily tasks is planned to start and end in two months. If tasks are scheduled During the rollups first month of duration and or scheduled in the second months duration for the 1st month of the task duration and nothing is planned and scheduled in the second month's duration the roll up task will not be 100% completed until the last day of the duration, hence roll up task will calculate 50% completed at this point even if all the tasks are completed.
If a task is a summary task whether the duration has ended or not the calculation of the summary task is based on the completion of the sub/daily task. If the 1st month has all planned tasks completed and there are no tasks that are incomplete in the second month then the summary task will also show as 100% completed.
Protip: To recap a Rollup calculates based on duration of the task, while a summary task calculates based on its subtasks completion percentages.
Protip: Summary task based on the average sub task.