最近在研究GSM通信模块相关的AT指令时，发现短信部分经常提到个短信的class，不知为何，搜了一下，特此记录：
What are the classes of SM-MT (mobile terminated) messages?
Classes identify the message’s importance as well as the location where it should be stored. There are 4 message classes.
Class 0: Indicates that this message is to be displayed on the MS immediately and a message delivery report is to be sent back to the SC. The message does not have to be saved in the MS or on the SIM card (unless selected to do so by the mobile user).
Class 1: Indicates that this message is to be stored in the MS memory or the SIM card (depending on memory availability).
Class 2: This message class is Phase 2 specific and carries SIM card data. The SIM card data must be successfully transferred prior to sending acknowledgement to the SC. An error message will be sent to the SC if this transmission is not possible.
Class 3: Indicates that this message will be forwarded from the receiving entity to an external device. The delivery acknowledgement will be sent to the SC regardless of whether or not the message was forwarded to the external device.
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