SAN certs

Standard SSL cert = web.domain.com

Wildcard cert = *.domain.com
Unlimited sub domains.

SAN cert = up.domain.com
                    to.domain.com
                    five.domain.com
                    in-these.domain.com
                    certs.domain.com
                   
Up to 5 sub domains
Generally used with phone systems
Some companies use them to cover their standard sub domains

webmail.domain.com
mx.domain.com
remote.domain.com