Tag button in the top menu allows you to assign and remove tags, search for existing tags and create new ones.
By selecting and de-selecting checkboxes tags can be assigned and removed to a task or a group of tasks at once (see Tasks Bulk Operations). This operation should be confirmed by clicking on Apply button, that will appear at the bottom part of the section after tags are modified:
Create New button allows you to create a new tag. You can create hierarchical tags using “/”, e.g: “Level 1/Level 2”:
Manage tags link opens Tags Management section for a Project or an Area.