A built-in hold is a period in a launch countdown during which no activities are scheduled and the countdown clock is stopped.
The hold serves as a milestone in the countdown, an opportunity for non-launch activities (such as a shift change or meal break), and a chance to perform unanticipated activities such as equipment repair.
Most importantly, a hold provides an opportunity to synchronize the pre-launch activity schedule (concluding at T−0) with the desired wall-clock time of launch (L−0).
A planned hold may be of a fixed or variable duration.
Criteria for exiting the hold and restarting the countdown may be based on a fixed time, the completion of a checklist of work items, or a go/no-go decision from mission management.