zone and sub-zone

[Frank Y.F. Luo]

a little confused about the relationship between zone and sub-zone. I know
zone and sub-zone are both separate file or DB.

But, for example:

host A hosts both domain xyz.org and sub-domain abc.xyz.org as master

host B only hosts domain xyz.org as slave, no sub-domain abc.xyz.org defined

Say a client looks up against host B for a host name 123.abc.xyz.org in
sub-domain. Since host B is also authoritative for xyz.org (but not
abc.xyz.org), will host B reply as NXDOMAIN or will go to host A find out
and return the right resolution?

Which part of RFC talks about this?

Thanks