The Add/Remove Projects/Tasks window appears when clicking on the Add/Remove Tasks button on the timesheet. This window is used to add or remove projects or tasks to or from a user's timesheet. Its Project filter typically includes an <All> option which lists out all the projects assigned to a user.
If a user is assigned over a 100 open projects, the <All> option in the Project filter of the Add/Remove Projects/Tasks window will be unavailable for that specific user. This is set up because displaying more than 100 projects and tasks may inhibit a drop in performance of the browser.
This restriction is incorporated in the application as the browser will not be able to handle it and thus would lead to slow performance when accessing the application. Users will have to make use of the Client and Project filters effectively under the Add/Remove Projects/Tasks window in-order to choose the right Task to add to their timesheet.