Activities
In order to be completed, a task may need different types of work to be done. Thus, a software feature can be designed, developed, and tested. Sometimes, information about the time that was spent on such types of work is very important, as it helps determine which work is more time-consuming and therefore allows optimization of the workflow. Also, such information is especially needful if different types of work involve different hourly payment rates. So, that's what the Activity field of the time entry should be used for.
Certainly, activities for time entries should not be defined by end users—their names and usage should be controlled by managers. Therefore, in Redmine, they can be managed only by administrators and only project managers (users who have the Manage project activities permission) can choose which activities apply to the project.
Administrators can manage activities using the Activities (time tracking) table, which can be found on the Enumerations page of the Administration...