About Primary Fields in Process Administration
You can use a picklist field on a record type to restrict a process so that the process applies only to certain records of that record type. The picklist field is referred to as the primary field, and a process applies only to records where the value in the primary field on the record matches one of the values that you specify for the process, that is, the primary values. For example, the Call Type field can be used as the primary field on the Activity record type. You can restrict a process for the Activity record type so that the process applies only to certain types of call records.