When creating an application form in Settings > Forms, the "Use as application name" option allows you to select a field which will be used as the application's name - this means it will appear on all lists for judging, public voting, etc. under that name.
The "Use as sub-title" option allows you to set a sub-title for the applications and functions the same way as the "Use as application name" option.
You can only select one field to be used as application name and one field to be used as sub-title. A field cannot be used as application name and sub-title at the same time.