On VM-Series software, what is the difference between Base and non-Base images?

In Palo Alto Networks Support Portal, the customer can navigate to (Updates > Software Updates) for downloading VM-Series software, but what is the difference between Base and non-Base images?
 

• Palo Alto Firewall VM-Series 
• PAN-OS for VM-Series

Below are the differences between base and non-base image:

1. "PAN-OS for VM-Series Base Image" is for deployment and installation. This image is used for new installation only
2. Non-Base "PAN-OS for VM-Series" image is the regular firewall upgrade file, and is used for upgrade of an deployed firewall. This image cannot be used for initial deployment.