Tags are independent of Projects, Tasks, and Clients and are totally customizable. The administrator can create a set of Tags that team members then use to mark their time logs. Or, the administrator can also tag time logs themself in the Activity view.
You could for example create tags such as 'Fieldwork', 'Remote work', 'Prepaid hours', 'Expenses', 'Flat fee task', ... You can create a Tag for any custom category that My Hours does not provide out of the box but you would like to track anyway.
You will be able to create and edit Tags if you go to the 'Manage your tags' section of Account admin.
Team members can then use these Tags to mark their time logs:
Or, you as an administrator can go to edit a time log from any team member in the Activity view and add a tag to it.
The Activity report can then be filtered by these tags to show you all time logs that belong to each category: