Agile Tasks: Task Type Code - how do I use this?

Under Agile Tasks if I select 'Choice Lists' I see the following lists:

Release Link Types

Priority Codes

Status Codes

Task Type Codes



I understand where Priority and Status fit in.

I can add some entries to Task Type Codes but I cannot find any way to cause a field to appear on a screen. 'Task Type' does not appear in the list of fields for any Agile Task screen.



The same question probably applies to the release link types but I am less interested in that. 'Task Type' sounds useful.