Data Type Yes/No (Yes/No field: A type of field with content set to either Yes or No; also known as a Boolean field or a True/False field. For example, the Recurring field indicates whether the task is a recurring task.)
Entry Type Entered (entered field: A field in which you may type or edit information. This is in contrast with a calculated field, in which Project calculates and enters information.)
Description For subtasks, the Rollup field indicates whether information on the subtask Gantt bars will be rolled up to the summary task bar. For summary tasks, the Rollup field indicates whether the summary task bar displays rolled up bars. You must have the Rollup field for summary tasks set to Yes for any subtasks to roll up to them.
Best Uses Add the Rollup field to the Gantt Chart view to set which subtasks are to be rolled up to the summary task bar. By default, all summary tasks are set to Yes and all subtasks are set to No.
Example You want important milestones to appear on the summary task. You set the Rollup field for these milestone tasks to Yes.
Remarks To use the Rollup field for summary tasks, select a summary task and then in the Task Information dialog box choose whether you want to show rolled up Gantt bars and whether to use the Rollup field for subtasks.
If you are rolling up several subtasks, you can also hide the summary task bar in Task Information dialog box.
You can globally set rollup behavior for all summary tasks on a Gantt Chart view in the Layout dialog box.