I have also been caught out with dimming switches, and one has to read the instructions, it seems there is no one diagram fits all. In basic terms we have. 1) Stand alone dimming switch. 2) Two way dimming switch for use with standard two way switch. 3) Master dimming switch 4) Slave dimming switch I think you have number 2 so either L1 or L2 with the AC wavy line with arrow through it for other wire of the standard two wire switch. If used with a two way switch then L1 and L2 go to standard two wires and L1 also goes to L1 on other switch, and L2 to L2 on other switch and AC wavy line with arrow through it to com on other switch.
However the master and slave dimming switches also have similar marking, with the master and slave system the third wire carries a signal rather than full current and you can have one master and many slaves all which can adjust the light.
Added to this is the wattage, old dimming switches would work with 40 to 100 watt bulbs, but today we have bulbs of 5W and the old dimmers do not work with such low wattage.
So even as an electrician I have to read the instructions that come with the switch. Some even connect to your phone to change the light level. There is no longer a one size fits all for dimming switches.
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