PushPayload里面的setSMS能解释一下是干嘛的吗

```
PushPayload payload = PushPayload.newBuilder()
.setPlatform(Platform.all())
.setAudience(Audience.registrationId(regId))
.setMessage(Message.newBuilder()
.setTitle(title)
.setMsgContent(content)
.addExtras(extras)
.build())
.setSMS(sms)
.build();