RFC 1035 says:
MNAME
The <domain-name> of the name
server that was the original or primary source of data for this zone.
although in practise this MNAME field in the SOA is mostly unused these days.
However if you’re using DNS dynamic updates then it must refer to the name of the DNS server which is to receive the dynamic update messages.
See also this (expired) Internet Draft which talks about the MNAME field in detail, and how the DNS UPDATE message is the only current use for it.