company, project, subproject, person, user, task, absence,... can be active or inactive.
Active means they can be used in a timesheet.
There is a Deactivate button in the modules:
Company details, Project details, Person details and User details.
For instance when a project is over.
Click Deactivate and nobody will be able to record time anymore on that project.
If you click the link the entity will be removed permanently, and this can't be undone.
You cannot delete entities that are used in someone's timesheet.
But you can leave it as inactive.
The entities Tasks, Subproject and Absences can be active or inactive as well, but it is slightly different.
For instance a subproject: