There may be instances where you would like to define planned effort and log time at a high level such as at a phase or milestone instead of each individual task. The "Time Tracking Only" option on tasks allow you to create tasks that serve as placeholders for time tracking purposes. When Time Tracking Only is checked for a task, it is not included in the roll-up fields on the project (i.e. Total Duration, % Complete, Start and Finish Dates, etc).
If you do not see the Time Tracking Only checkbox in the Project UI and you do not see it as an option to add as a column, please see Add Time Tracking Only Option in Project UI.
Create your project plan as you normally would. However, for Time Tracking Only Tasks, we recommend creating them as separate section in your project so that they are not included in the roll-ups within your hierarchy (i.e. parent tasks).
In the example below, the time tracking tasks are in a separate section at the bottom of the project.
Please make sure that you check the box for "Time Tracking Only" so that they are excluded from the roll-ups on the project.
You can choose to specify Duration and Planned Effort for your Time Tracking Only Tasks. Time Tracking Only Tasks are excluded in the Resource Allocation page when plotting a resource's allocation and capacity.
You will then need to assign the Time Tracking Only Tasks to your resources so that they are able to log time against it via Time Tracker.