Parent activity's estimated hours is a total of its own estimated hours, if any, and those of its sub-activities.
Activity's own estimated hours are given in the 'Estimated hours' field () in Activity details section.
The total estimated hours of a parent activity and its sub-activities are given as the total estimated hours under the time usage bar of the parent activity (see Activity details for details) before any actual hours are recorded. For difference between estimated hours and hours left, see here.
An activity 'Move office' has 6 estimated hours, and two sub-activities: 'Move furniture' and 'Move equipment', 3 and 5 estimated hours, respectively.
The total of estimated hours in this case is T = 6+3+5
This amount will be shown as "14 estimated hours" under the time usage bar in the details section of the 'Move office' activity before any actual hours are recorded.