Each custom question you create for your registration forms can be added to your library. When editing a registration form you can:
Go to the Custom Questions Section
Click the Manage Questions Library button to the right
Importing questions lets you add custom questions from previous forms to your library
Add a question directly using the 'Add Question to Library' button
After adding your custom questions to the library, you can see them when creating a new custom question on any future forms. Under the first dropdown they're found under 'My Library'.
By using the Custom Questions Library, it will save you time on any future forms being created!