The A record is what links your dedicated IP address to your chosen subdomain. Creating this record will, for example, point traffic for o1.subdomain.yourdomain.com back to your dedicated IP, and vice versa.
You can create multiple A records for a domain with as many IP addresses as you wish, however you can only whitelabel one IP to one domain at a time.
What it should look like:
em.yourdomain.com. | A | 192.168.250.1