# Understanding Amazon Chime SDK message types
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You send messages through channels. You can send `STANDARD`, `CONTROL`, or `SYSTEM` messages.
+ `STANDARD` messages can be up to 4KB in size and contain metadata. Metadata is arbitrary, and you can use it in a variety of ways, such as containing a link to an attachment.
+ `CONTROL` messages are limited to 30 bytes and do not contain metadata.
+ `STANDARD` and `CONTROL` messages can be persistent or non-persistent. Persistent messages are preserved in the history of a channel and viewed by using a `ListChannelMessages` API call. Non persistent messages are sent to every `AppInstanceUser` connected via WebSocket.
+ Amazon Chime SDK sends automated `SYSTEM` messages for events such as members joining or leaving a channel.