You may need more than just personal information on attendees or the registration contact, and you can gather that information by asking questions. Answers will be saved to the attendee's registration form.
Check out this clip from Planning Center University.
Choose to add questions for the attendee or the registration contact to answer, and then click Add a question.

Set up the question as you want it to show when someone's registering.

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Enter the question in the way you want it to be shown on the event page, and then enter options, if applicable.
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Choose the type of answer you want to receive.
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Show how you want this question to be shown on Church Center.
If the Required box is checked, an attendee cannot complete the registration until it's answered.
Click Save to add the question to your event and view a list of all other questions.
Use a section to split up your questions. Click New Section, and then enter the name and description.

Drag the section above the questions it applies to.

Add more questions or section headers or click a question/section to edit or delete it.