How to determine which information (email, phone number, etc.) is mandatory for your guests and which is optional
Setup the booking / inquiry form
Navigate in the CMS to Hotel/Special settings. Here you will find the settings for the booking form. Using the tabs, you can determine which fields should be displayed and whether they are required, i.e. must be filled out by the guest. 
Add custom fields
Via the last point "Add custom field" you can add further fields to your booking/inquiry form and name them individually.