> defpsudo    xxx: ifnet ether

> In the above line, what does the meaning of the "ifnet" and "ether" ?

Nothing, on their own.

ifnet is a device class ("devclass ifnet" is present), which means that
the device (or pseudo-device, since you asked about a defpseudo line)
in question has type DV_IFNET.  Presumably there can be code in the
kernel that does something noteworthy for such devices (eg, root device
specification - root on a DV_DISK device should try mounting a
filesystem like FFS; root on a DV_IFNET device should try BOOTP/etc).

ether is just a device attribute ("define ether").  AFAIK it's used by
lines like

file net/if_ethersubr.c                 ether | fddi | netatalk | token

which includes net/if_ethersubr.c in the kernel if any "ether", "fddi",
"netatalk", or "token" attributes are indicated for included devices or
pseudo-devices.

Of course, the real final reference for what the various things in
config files mean is /usr/src/usr.sbin/config/.

> I looked into  the "Device Configuration in 4.4BSD" paper, but I
> couldn't find the information about that. Is there any material about
> that?

ISTR hearing that our config(8) has moved a nontrivial distance since
that paper was written.  If true, this could explain the lack of
information you found.

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