Depending on the class or service that a client has booked, you can ask the client to fill out specific fields related to the class or service they are booking.
First, you will want to create the set of custom fields under Setup → Custom Fields. Click this link to read the documentation on creating custom fields.
Then you will want to apply those custom fields to the respective class or service that they relate to. Click here to learn about applying custom fields to a class or click here to see how to apply them to a service.
Finally, you may want to display some or all of the custom field answers in your email messages or on the confirmation screen. View our documentation on creating dynamic tags for displaying the answers to the custom field questions