Cold backup is a database backup taken with PPM services stopped. Usually this happens during maintenance hours on weekends.
Hot backup is taken with app services running and users connected, so there is a chance that some of the data is modified in the meantime, which could lead to different mismatches.
CA Support does not support hot backups for upgrade. For all activities other than disaster recovery a cold backup should be used. This includes migrating environments, upgrading, changing hardware etc.