Compulsory field means that you Must fill in the field details otherwise you can't import other information (email, telephone, etc.) as well.
What is the compulsory field when importing the attendees?
Answer: FULL NAME
You Must enter the full name and so as can enter other information:
Even though you've got the email or telephone number of the attendees, you still can't import and update the attendee information.
Therefore, full name must be entered before you import other information.
The following will show that what will happen when there's Only entering the FUll NAME.
It showed that the attendee was updated even though other details (telephone, email, company, salutation) were not entered.
Therefore, Full Name must be entered before you import other information.