What's the difference between DB_HOST and DB_HOST_LOCAL_SUBNET?

I'm trying to create a new clean staging DB. However, I can find these settings in .env which match two RDS instances on AWS.

What's the difference between the two and do I need to create two RDS instances for the new staging environment?

DB_HOST=companystaginginstance..us-east-1.rds.amazonaws.com DB_HOST_LOCAL_SUBNET=companydbmysqlstaging..us-east-1.rds.amazonaws.com