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In a 3-phase power environment, you can assign one complete line to L1, the next line line to L2 and the third line to L3. By switching all line separately on and off, this system could be used as a crude dimming function.
You can also distribute L1, L2 and L3 on one single line. By switching all lines separately you got a dimmable system again.
On the other hand, you can use L1 and L2 ad power lines while L3 servise as emergency power line connected to a central power supply.
When the power cuts out, the central emergency power supply kicks in and switchs on all fixtures connected to L3.