When you create a subtask on an existing issue, you have to specify a subtask issue type.
Unfortunatley, all possible subtasks (in the current Issue Type Screen Scheme) are shown...
Would it be possible to make the subtask selection dependant on the task issue type?
For task TA you would be able to select subtask SA, SB, SC, and for Task TB you would be able to select subtask SB, SD, SE and SF.