Nothing, however Bark detects changes in activity over time and notifies you if one of your child's accounts has significantly less activity as compared to previous periods. This drop-off often indicates the child may be using a different account on that same platform, or perhaps using a new platform altogether. Either way, it's worth asking the child about their change in usage and understanding why it might be the case.
When your child signs up for a new platform, there is generally a text or email that is sent to an existing phone number or email address to confirm that sign up. If your child's iOS or Android device is connected to Bark for text monitoring or your child's email address is connected to Bark for email monitoring, Bark will detect this message and alert you of a "new account detected".