Use of network_interfaces in rc.conf

[hiren panchasara]

Looking at man rc.conf,

     network_interfaces
                 (str) Set to the list of network interfaces to configure on
                 this host or “AUTO” (the default) for all current
interfaces.
                 Setting the network_interfaces variable to anything other
                 than the default is deprecated.  Interfaces that the
adminis‐
                 trator wishes to store configuration for, but not start at
                 boot should be configured with the “NOAUTO” keyword in
their
                 ifconfig_⟨interface⟩ variables as described below.

For example, in my rc.conf I have

ifconfig_em0="dhcp"

What difference does it make when I have each (separately) in my rc.conf:

1) no network_interfaces at all
2) network_interfaces="AUTO"
3) network_interfaces="em0"
4) network_interfaces="NOAUTO"

It seems, all but 2 are deprecated.

Thanks in advance,
Hiren