In 2013 and 2014, a Russian/Ukrainian hacking ring known as "Rescator" broke into Target Corporation computers in 2013, stealing roughly 40 million credit cards, and then Home Depot computers in 2014, stealing between 53 and 56 million credit card numbers. Warnings were delivered at both corporations, but ignored; physical security breaches using self checkout machines are believed to have played a large role. "The malware utilized is absolutely unsophisticated and uninteresting," says Jim Walter, director of threat intelligence operations at security technology company McAfee – meaning that the heists could have easily been stopped by existing antivirus software had administrators responded to the warnings. The size of the thefts has resulted in major attention from state and Federal United States authorities and the investigation is ongoing.
What machines were involved in the physical security breaches that played a large role in the incident?
self checkout machines
The phrase "physical security breaches" can be interpreted to mean a physical attack against the computer systems. In this case, it is stated that these attacks involved self checkout machines and played a large role in enabling hackers access to the system. 
Therefore, based on the context of this paragraph, we can conclude that these were physically breached causing them to become vulnerable for hacking into Target's and Home Depot computers.".