# Understanding WebSocket system messages in elastic channels for Amazon Chime SDK meetings
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The Amazon Chime SDK sends system messages to all connected clients for events that take place in channels. The following list describes the system messages for elastic channels.

**Message events**  
Event payloads for elastic channels contain the `subChannelId` field. Payloads for non-elastic channels remain the same.

**Membership events**  
The `CREATE_CHANNEL_MEMBERSHIP` and `DELETE_CHANNEL_MEMBERSHIP` events now have the `subChannelId` field in their payloads.   
Elastic channels don't support the `BATCH_CREATE_CHANNEL_MEMBERHSIP` event. When you call the [https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_BatchCreateChannelMembership.html](https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_BatchCreateChannelMembership.html) API, the system sends individual `CREATE_CHANNEL_MEMBERSHIP` events.  
You can now use the `UPDATE_CHANNEL_MEMBERSHIP` event type to signal changes in membership information. For example, during a member transfer from one sub-channel to another, the system sends an `UPDATE_CHANNEL_MEMBERSHIP` event with the new `SubChannelId` in the payload to indicate that the member was transferred.   
The system only sends the `UPDATE_CHANNEL_MEMBERSHIP` event to the member that was transferred, and not to other members of the sub-channel. For this reason, we encourage you to use the [https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_ListChannelMemberships.html](https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_ListChannelMemberships.html) API instead of WebSockets to populate your channel membership rosters. For more information, refer to [Using WebSockets to receive messages in Amazon Chime SDK messaging](websockets.md).