Thank you very much for your support!

You may add a tag when editing a task. Click 'tag' column in task detail page, add a new tag if the existing ones are not suitable for this task.
But deleting a tag can only be done in web version: https://i.doit.im/home/#/settings/tag .
We'll add the feature in iPhone later.

If any further questions, please feel free to tell us.
Have a nice day!