The Primary Carer is the Care Professional in charge of delivering the care to the Senior at home (wherever home is). He or she is responsible for the service to the Senior. In some countries (the United States for instance) they are called Caregivers, in others (like the UK), they are called Carers or Care Workers.
It is important to indicate the Primary Carer of a Senior for 2 reasons: (1) to be able to track the Quality of Service delivered by the carer to his or her Seniors, (2) to give carers access to the Senior profile and assessments.
Indeed, Care Professionals that are not Administrators can only access the profiles of Seniors for which they are Primary Carers (read, Administrators vs. Regular Care Professionals).
To set a Primary Carer for a Senior, he or she needs to exist as a Care Professional in the SeniorGrowth database (see article, How to create a Care Professional?)