There are 2 reasons why fields cannot be deleted from Register Management.

1. System Fields: There are a small number of System Fields within each register, such as Name, Status and Description which are required for each register.  These fields cannot be deleted, moved or hidden; however, the field Name can be edited. 

2. Fields Used In a Master Record Privacy Assessment: Some of the fields in registers cannot be deleted if they are already being used within a Privacy Assessment, e.g. either system templates or those created from scratch by customers.  If you see the message below appear, you will know the field you are trying to delete is being used in a Master Record Privacy Assessment.