CNAME record indicates that the domain is alias other domains in the sense that they have a common DNS record (not common web content, etc.).
If for the alias domain exists this record, it shall not exist any other. Example:
www1.domain.cz. 1800 IN CNAME domain.cz. www2.domain.cz. 1800 IN CNAME domain.cz. www3.domain.cz. 1800 IN CNAME domain.cz. domain.cz. 1800 IN A 184.108.40.206 domain.cz. 1800 IN MX 10 mail.domain.cz
Here are subdomains www1, www2 and www3 as aliases for domain.cz and all of them have IP address 220.127.116.11 and also the same MX record. The sense of CNAME record is evident – have a real IP address and any other records only in one place and to change them is not necessary to rewrite all domains separately.