You can define mandatory variables when you want some information to be always filled for each patient.

To make a variable mandatory, edit the variable and uncheck the parameter "Is optional".

When adding or editing a patient, mandatory variables will be marked with an asterisk * to inform users they are required. 

A missing value for a mandatory variable will not prevent any user to add the patient but the patient will not be displayed as 100% completed until all the mandatory variables are filled.