Ticket tags are a way to label and categorize tickets and are independent from the ticket form, ticket type and ticket queue. In other words, they are not dependent on anything else and can be added to tickets as needed by agents. Ticket lists can be filtered by tags and also triggers can be set up to take action on tagged tickets or to automatically add tags to tickets based on specific criteria.
Managers and admins can create ticket tags by going to Admin Settings > More Settings > Tags.
Or, tags can be created from the ticket itself by clicking 'edit tags' from the settings menu on the ticket details interface (3 vertical dots top right of ticket page). This is also where existing tags can be added to tickets by agents, managers or admins.