There are different custom field inputs which can be used.
Using a certain input type will control what information can be entered into the custom field.
Check box which is generally used for a yes or no answer
e.g. “Do you have existing medical insurance?”, Yes OR No
Date input, the field can be clicked and a date selector is shown
e.g. “Your mothers date of birth”, 1960-01-01
Standard text input box but a valid email address must be entered
e.g. “Home email address”, email@example.com
An input box where only numbers can be entered
e.g. “Years of experience”, 8
Single text input
Standard text input box which is one letter tall
e.g. “Nickname”, Daz
A text input box but multiple lines can be added
e.g. “Employment references”