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and I think if you read the note * for Izm the values depend also on the temperature so I think the Vz and Izm are values calculated under 400 mW disipation conditions.
You are worrying without any reason! - the 500mW is a function of the package, NOT the actual zener. All the zeners in the range will have a rating of 500mW (as they use the same package), the restrictions imposed by the zener voltage may, or may not, allow you to reach that figure - on the higher voltages it may be possible to exceed it!.
Don't worry about it - with anything like this, work out the dissipation from the voltage and current, and make sure it's BELOW the maximum dissipation, preferably well below!.
Power = voltage times current.
The choice of zener diode determines the voltage. Your circuit's requirement and your choice of unregulated supply voltage, your selection of the value of the current-limiting resistor, and the zener's load determine the current.
The actual voltage across a zener diode depends on the current through it. At currents far below the amount stated in the datasheet (10%?) a zener diode regulates poorly and the actual voltage will be much too low.
Only zener diodes with voltages of between about 5.6V and 6.8V are temperature stable.
The others change their voltage with temperature changes. The voltage stated in the datasheet is when the zener diode has warmed-up by its dissipation due to the current it is tested with. Therefore their voltage will change as they warm-up and continue changing if the unregulated input voltage or load current changes.
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