INTERFACE and INTERFACE_MACRO explanation

Can anyone make head or tail of the explanation of INTERFACE and
INTERFACE_MACRO in perlxs.pod?  I've read and reread it several times
and it still doesn't entirely make sense.  what on earth is "symbolic" -
a reserved xsubpp word or just a placeholder?  Could someone translate?

Alan Burlison

• INTERFACE and INTERFACE_MACRO explanation by Alan Burlison
• Re: INTERFACE and INTERFACE_MACRO explanation by Brendan O'Dea
• Re: INTERFACE and INTERFACE_MACRO explanation by Nick Ing-Simmons
• Re: INTERFACE and INTERFACE_MACRO explanation by Brendan O'Dea
• Re: INTERFACE and INTERFACE_MACRO explanation by Jarkko Hietaniemi