DNS and domains
CNAME record
A DNS record that makes one name an alias for another. Used to point www or a subdomain at a hosting provider without copying its addresses.
A CNAME says "this name is the same as that one": www.example.lu CNAME s-123.sites.example. A resolver that asks for www.example.lu is told to look up the target instead, and gets whatever addresses the target has. When the provider changes its addresses, your record keeps working.
The rule that catches everyone: a name with a CNAME may have no other records. That makes a CNAME impossible at the bare domain (example.lu), which must hold the zone's SOA and NS records. For the bare domain you use A and AAAA records, or an ALIAS/ANAME record if your DNS host offers one; those behave like a CNAME but are resolved by the nameserver itself.
CNAMEs are also what mail providers use for DKIM keys and for the autoconfig host, so a mail client can find the server by name.