The difference is primarily where the application is hosted. With Monthly Plans, the application is hosted on YouTestMe servers and in that case, you are renting the application.
As about Software Purchase, you become the owner of application and you decide whether you will host the application on our cloud servers or on your on-premises.
Please note that if you decide to host the purchased application (GetCertified or Classroom2020) on our cloud server, you pay the monthly cloud hosting fee, starting from US$99/month. You are eligible to migrate the application at any time, free of charge.