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What you want is to create multiple custom msgs (for how to create custom msgs, see @JackB answer).

Then, you need to nest the messages the way you want. Something along the line of the following msg definitions:

BatteryStatus.msg:

float32 current
float32 voltage

SystemStatus.msg:

int32 error_code
string error_msg

FullSystemInformation.msg

BatteryStatus battery
SystemStatus system

And then you can access the fields like, e.g. msg.battery.current or msg.system.error_code.