Kimai supports tagging timesheet records, which allows you to create arbitrary logical structures.
Users can assign any number of tags to each timesheet entry, which you can use to filter records in exports/invoices and reports.
There is a tag administration screen, which displays all available tags and the counter how often they are used.
The action menu for each tag allows you to filter all records for the choosen tag, by linking to the team timesheet with a preset tag filter.
When a tag is deleted, only the tag and all links to it are deleted. The linked timesheet records are NOT touched.
You can query for a free search term, which will be searched in the fields: