Basically, there are four SMS classifications, while the regular SMS message is classified as a Class 1 message, flash SMS messages are classified as Class 0 messages, which classifies them as most important SMS messages. The difference is that Class 1 messages are stored on users' mobile phone SIM card or phone memory (depending on memory availability) by default.
On the other hand, Flash SMS messages (Class 0) appear directly on the phone’s screen without any user’s interaction/action and are not automatically stored in the inbox. Recipients have the option of saving these messages, with the appropriate sender ID, if they choose to, but it’s not done automatically.
Very importantly, where multiple flash messages are sent, it's vital to keep in mind that users will only be able to see the last message they receive. In other words, users can only have one FLASH message displayed on the screens of their phones.