Zero Fuel Weight (zfw)

[betelgeuse 41]

I've just started flying the A320 and have flown the tutorial flight several times. Most things I've been able to understand but ZFW has me beat! In the tutorial a value of 59.4 (59400 kg) is taken notionally as a reasonable figure for that flight from Bristol to Grand Canary. But what does ZFW represent? More importantly, how does it relate to the values in the default payload/fuel loader in FS2004? The values in that are what counts and if they do not match the FMC input odd things will surely happen.If I plan a flight, say from EGFF to EGAA (1 hour flying) with all seats taken, what value (kg) should I enter for payload in the FS2004 fuel loader? What value then should I enter for ZFW in the FMC?John