Documentation for JIRA 4.2. Documentation for other versions of JIRA is available too. 
By configuring a custom Issue Type Screen Scheme, it could able to have a specific screen for sub-task issue type. For example:
1. Create a screen via Administration -> Issue Fields -> Screens (e.g. Sub-task screen)
2. Create a Screen Scheme via Administration -> Issue Fields -> Screen Schemes (e.g. Sub-task Screen Scheme)
3. Configure this newly created screen scheme to have a 'sub-task screen' when creating issue
4. Create a Issue Type Screen Scheme via Administration -> Issue Fields -> Issue Type Screen Schemes5. Configure this newly created Issue Type Screen Scheme to have a 'Sub-task Screen Scheme' for 'sub-task' issue type
6. Associate this Issue Type Screen Scheme with the project
For more information on Issue Type Screen Scheme, please refer to this documentation:
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