CPIN i.e. Common Portal Identification Number is a 14 digit number generated when a challan is created before payment of GST. CPIN is used to identify the GST Challan.
CIN i.e. Challan Identification Number is a 17 digit number generated when a challan is paid. Thus, CIN is generated after payment of GST. It comprises of 14 digit CPIN and 3 digit bank code. CIN is used to track the status of GST Challan, whether paid or unpaid.
Read more - Process to pay GST Challan online